Although I agree it's comforting to know many others are having the same issue, what I really want to know is what the deal is. Why is this iPhone 4 specific? I've heard plenty assertions that the 3G network gets jammed up with heavy loads and consequently drops calls, but that doesn't seem to be the culprit here since the 3GS or any other AT&T phone that uses 3G doesn't drop EVERY 3G phone call (as is the case with my iPhone 4). Conversations on Edge don't seem to drop quite as often (rarely) but my phone calls are guaranteed to drop while the phone is using the 3G network within 60 seconds to a few minutes of initiating a call. For the most part, switching between 3G and Edge is fine as a temporary solution, but it can get quite annoying when 3G is switched on for internet browsing and a phone call comes in. My options at that point are either to ignore the call, switch to Edge, and call back OR answer the call, speak for a bit, drop the call, switch to edge, and call back (both options suck).
As many people have already mentioned before me, it doesn't matter where I am or how I'm holding the phone, 3G phone calls just don't hold. When placed side by side with a 3GS and phone calls are made from both phones simultaneously, the iphone 4 drops out (consistently) while the 3GS stays connected without a hitch.
I'm not sure why this hasn't been publicized nearly as much as the antenna issue considering this is a MUCH bigger problem that cannot be resolved by merely placing the phone in a case... what are the chances that this can or will be addressed in a software update?