b) Yes, I can recreate the signal drop issue (although I can't get it to drop to zero).
c) No, I don't consider it to be a fatal flaw.
a) I bought one on launch day after reserving it back on June 15th. Not much else to say there.
b) By palming the phone in my left hand, I can get the signal bars to drop from five bars to three bars. I've also performed speed tests to compare 3G speeds when held differently. Here are those results (using the Speedtest.net app):
Held in Right Hand: 4.14Mbps down, 1.38Mbps up, 396ms ping
Held in Left Hand: 3.16Mbps down, 0.49Mbps up, 1208ms ping
So yes, palming the phone in the left hand has a measurable impact on signal quality.
c) I didn't bother doing this test until I'd actually heard this was an issue, because I simply didn't notice it. The first thing I did with my iPhone when I had it out of the box and was still in the Apple Store was put it in a bumper case.
When I'd heard there was an issue, I asked my girlfriend if she'd had any issues, but she also didn't notice it. Her phone isn't in a case, but she holds her phone in her right hand.
Once it was isolated to the bottom left corner on the MacRumors forum, I took my phone out of the case, palmed it, thought "huh, interesting", ran speed tests, then put my case back on and went on about my business.
So yeah, the issue is a non-issue if you are right handed, or if you are going to put your iPhone 4 in a case.
Now, I realize that that isn't going to be good enough for most people here on slashdot. Apple is putting form over function, it's unreasonable to expect people to hold their phone differently, etc, etc, etc. Fair points all around.
But I honestly am not bothered by it. I was planning on buying a case anyway, and even if I weren't, the vast majority of calls I make or take are done with the included headset (meaning I'm not touching the phone at all). Plus, the phone is still a noticeable step up in speed, battery life, screen quality, and reception (even despite the "fatally flawed" antenna) over the iPhone 3G it replaced.
YMMV and all, but I think this issue is being blown out of proportion. It's a flaw, but it's not a dealbreaker. It's at worst a minor annoyance on what is, to me, still the best smartphone on the market for what I want/need in a phone (sorry Android, you've come a long way since the G1, and I dig the Evo's giant screen, but iOS4 is still better polished and the App Store still has more quality apps). All of that last paragraph is IMHO, of course, and is worth only the requisite $0.02. Not a cent more or less.