Both ignorance and misinformation abounds about the coming switchover to HDTV in the US. I will cut right to the facts.
Right now all major TV stations have two transmitters, their old/original, and a new, secondary for their DTV broadcasts, which is always lower power, on UHF. As of February 2009, they will be shutting off one of the two, and broadcasting digital-only. The FCC has allowed them to select which of the two they wish to continue using. At this point, it's largely settled. VHF-low is going away.
Practically all broadcasters on VHF-low (channels 2-6) are dropping their existing channel assignments, and switching to a higher (UHF) one. There are very few exceptions. There are also (much lesser) signs of broadcasters leaving VHF-high (7-13), but for the time being, there are lots of VHF-high channels, so you will need an antenna that can receive channels 7-13.
Also important is the fact that ATSC doesn't handle multi-path interference (ghosts) very well. Unlike current analog NTSC, it won't just make the picture look worse, if it's significant, it will prevent you from getting a picture at all. If you are in a city with tall building or big hills/mountains around, this is very important in the selection of an antenna. Older and more common (rectangular) VHF antennas are less directional, and can do little to suppress multi-path interference.
Also important: The FCC is removing UHF channels 52-69. There will no longer be TV broadcast in that range.
STRONG SIGNAL (0-20 miles):
If you are just a few miles from all the TV stations you want to receive, and there are no obstructions in the way, you would probably do well to just get a TV-top antenna that does both VHF and UHF (rabbit ears for VHF and a loop or bow-tie for UHF).
MEDIUM SIGNAL (20-40 miles):
If you use an older rectangular VHF antennas, multi-path interference on VHF signals can pose a problem. Don't get an antenna that is large, and/or highly directional with high gain (db) or else it will be unnecessarily difficult to install and aim the antenna. Hopefully all the channels you want to watch are coming from the same general direction, otherwise, you'll need a (low-end) antenna rotator to get a good signal or possibly two antennas pointed in different directions. I suggest a Winegard 4400 (4-bay) for UHF, and possibly an AntennaCraft Y5-7-13 for VHF-hi. To connect them, DON'T USE A REGULAR SPLITTER or you'll ruin your reception for no good reason. You want a $2 UHF/VHF combiner, commonly made by Pico Macom/Blonder Tongue/Holland/etc.
WEAK/FRINGE SIGNAL (40+ miles):
The most popular type of UHF antennas, yagis with corner reflectors (they look like big metal arrows), design is such that they perform poorly at lower UHF channels, and best at the highest UHF channels. One of the antennas I was considering buying only starts to perform well at UHF channel 50, channels which won't be around in a year.
With that in mind, I recommend an 8-bay antenna instead... One of the top performers which is also far less expensive than other 8-bay antennas is the Winegard 8800. 8-bays perform quite well at the lower range of UHF frequencies from 14 to 50. A few 8-bay antennas claim to get good VHF performance, but it's all misinformation (which I also fell for). At best, they only barely outperform the most basic rabbit ear designs, If you're far enough from UHF stations that you need an 8-bay, you'll also be far from VHF stations, and need a GOOD VHF antenna to bring in the signal. A barely-capable-of-VHF 8-bay simply doesn't have enough gain to get decent VHF reception. A $20 VHF-high antenna will blow them all away. Use a VHF/UHF combiner, NOT a standard splitter to connect them.
VHF-hi needs much smaller and cheaper antennas, and VHF-low is almost gone, except in Alaska, where stations are keeping their low VHF stations due to terrain.
There's one other advantage that multi-bay antennas have over yagi/corner reflectors. If you are renting an apartment, you are allowed to install any antenna you want, provided that it resides entirely on your property. Being able to mount a long yagi/reflector without it sticking out (no longer your property) is pretty unlikely, unless you have a huge balcony. Multi-bay antennas are quite flat, and even just an outside window ledge could be used to mount it legally. Just make sure it's mounted securely, because they are heavy. If there's no location for installation outdoors, you could also install it on the inside of a window, facing out, perhaps hidden from view by your curtain... a yagi would have to stick out, halfway across the room.
There's no substitute for putting your TV antenna on a nice long pole, hooked up to your HDTV tuner, and walking around your house with it until you find a location that gets a strong signal on all the channels you care about. A simple procedure, but a lot of time-consuming work. An antenna can be mounted up to 12' above the top of your roof without requiring you to apply for any permits, and in general, the higher your UHF antenna is, the stronger the signal.
Stay far away from power lines. Be sure to ground both the antenna pole and the coax cable outside of your house to prevent lightning damage.
Large multi-bay antennas are very heavy, so be sure to get heavier gauge pipe for mounting it.
If you are in a fringe area, and your cable runs aren't pretty short, you'll need a mast-mounted signal preamplifier. Lower noise (2.0db) is better, and only slightly more expensive than the rest. Gain (eg 15db) will hardly be an issue UNLESS you have extremely long cable runs... Only then, a second amp might help.
I'm looking forward to HDTV, not for the resolution, but because 95% of the shows I care about are on broadcast TV, and signal quality (fringe area) is the only reason I consider cable/satellite. DVRs, Netflix and Hulu obsolete most of the need for cable, and the sudden and total deterioration of the quality of original programming on the major cable networks sealed pay-TV's fate, IMHO.
If you still have any questions about the basics of digital TV, feel free to leave a comment, and I'll get to it. Coming up next time (soon!) similar tips for satellite reception...
FCC DTV tentative frequency assigments: http://hraunfoss.fcc.gov/edocs_public/attachmatch/DA-06-1082A2.pdf
Antenna performance comparison: http://www.hdtvprimer.com/ANTENNAS/comparing.html
Winegard UHF 4-bay 4400: http://www.solidsignal.com/pview.asp?p=hd-4400&d=winegard-hd-4400-4-bay-uhf-prostar-1000-high-definition-tv-antenna-%28hd-4400%29
Winegard UHF 8-bay 8800: http://www.solidsignal.com/pview.asp?p=hd-8800&d=winegard-pr8800-8-bay-uhf-prostar-1000-high-definition-tv-antenna-%28hd-8800%29
AntennaCraft VHF-hi y5-7-13: http://www.solidsignal.com/pview.asp?p=y5-7-13&d=antennacraft-by-radioshack-y5-7-13-highband-broadband-vhf-yagi-tv-antenna-for-channels-7-13-%28y5-7-13%29&sku=716079000987
AntennaCraft VHF-hi y10-7-13: http://www.solidsignal.com/pview.asp?p=y10-7-13&d=antennacraft-by-radioshack-y10-7-13-highband-broadband-vhf-tv-antenna--%28y10-7-13%29&sku=716079000994