Considering getting an iPhone 4, not sure as of yet though.
Let me just point out that again, we're relying a little too much on statistics and how numbers look on paper (or in this case, on a screen). So far, it looks like the majority of respondents don't have a problem, but even if 99% of the people responding say "no problems" or "not returning," that doesn't help you in real-world terms if you end up mathematically falling into the category that has huge problems.
The only way you'll know for sure is to try it for yourself. You've got 30 days to give it a shot. If you're coming from another carrier, don't port your number over right away. Let AT&T assign you one of their numbers, and then try the heck out of the iPhone 4.
If you have problems with dropped calls or lost reception, you should know pretty quickly, and can then return the phone and get a refund,
strenuously pointing out why you're returning the phone to the place you bought it from so that Apple hears about it. Your old number will safely stay with your old carrier, and you'll be none the worse for the experience.
But if by day 25 or so you're still happy, then you can give AT&T customer service a call, or visit one of their stores, and THEN tell them you're ready to port your old number over to the existing line. The AT&T-assigned number will go away, and will be replaced by the number you're porting in. Or if you'd rather keep the AT&T number and ditch your old number, you can do that too.
One last note: Making bars drop when you hold the phone a certain way is nice and all, but there are three metrics that you should pay attention to:
1. Whether the bars drop all the way down to the point where you lose service and the iPhone 4 starts "searching."
2. Whether your calls are dropping, whether text messages are not going through, or whether you are unable to make or receive calls.
3. Whether data is stalling or not responding.
If none of the above are happening, then you're fine.