One thing that I haven't seen mentioned yet, that might lend some mild credence to this issue possibly being software related (or fixable, or maskable) versus strictly a hardware issue is this:
Suppose the iPhone is programmed in a way that it stops trying to make a call and push data through its antennae when it "thinks" it doesn't have enough signal to push that data through... and furthermore it "thinks" it doesn't have enough signal because of the way the OS is programmed to report the signal measures from the antennae to the iPhone brain.
I could be way off, and i have no inside scoop or info to know if this is even something that is plausible, but it might provide some type of reasoning as to why its happening for some and not for others. Clearly at this point everyone is using the same iPhone 4 (or so we think) with the same iOS4.0 but perhaps some peoples' oilier/wetter hands do a better job at shorting out the signal (When its not already at or near 100%) and thus brings it down into weak range, which the OS hyperbolizes as "nonexistent range" and the iphone gives up hope that its got enough of a signal to do anything useful and in an effort to conserve and optimize battery, it just drops your call or halts your data throughput. The alternate would be to try even harder (read: use more battery) to push that data through, but knowing Apple and their battery preservation methods, the former seems more likely to be something they would implement.
If this is something along the lines of whats going on here, then they can just fix this by telling the phone not to report such low signal levels, aka, fixing a hardware design flaw with a software patch. Yeah its kinda masking the issue, but if it gets calls to not drop and data to keep working, I say DO IT! That's, of course, assuming that the iphone really does "give up hope" when it's told the signal is low, versus when it's actually too low to function.
This could explain why steve thinks it's a non issue. This could also explain why people who have an iPhone 3GS are now experiencing problems that they weren't before. This could explain Apple's relative silence on the issue - knowing that they are internally working on a patch that will fix this and some other stuff. This could explain why some iPhone 4 users experience the issue and others don't... AND it could explain why some users experience the issue sometimes but not all the time.