If you're writing code, you need a platform that is well supported by your IDE. If you're writing for vs.net all the time, you probably don't want a Mac. Using Eclipse for Java work, then Mac is fine -- etc.
Since almost any product on the market will work from a power standpoint, look at the details of form. Is the case well made? A magnesium or aluminum case can mean less flex even with less weight. Consider the touch pad -- is it multi-touch? Is the keyboard comfortable?
Also, watch the resolution. One mistake I've made in the past is getting too high a resolution screen for my eyes. At 15" the best resolution for my eyesight is 1440x900, so having a higher res screen means the typeface is too small or it's fuzzy as I switch to a non-native resolution for the screen (windows does NOT cope with rescaled fonts well).
In terms of stability, reliability, and so on -- I find Acer and Gateway to be near the bottom of the line; ASUS makes great hardware but I've never been happy with their support or documentation and their software (for custom bits of hardware, bios updates, etc) is downright terrible. Dell makes some great stuff in the latitude line, but the inspiron stuff isn't well made Dell's support has been downright misleading to me on more than one occasion (documented and published). FWIW, My Latitude D820 has been outstanding even if Dell's support has been terrible. HP has some stuff out that looks pretty, as does Toshiba but neither appeals to me all that much.
I'm kind of in the same boat as you -- I'm ready to replace this D820 after nearly 4 years, but nothing on the market right now really impresses me. I'm waiting for this winter's new stuff based on Core i7 to see what that looks like in a laptop. I'm also going to evaluate Windows 7. If it's not substantially more comfortable and more maintainable than Vista, I'll have no choice but to switch to Mac.