No that's a bug in the OS. The signal is still useable. Apple told Walt Mossberg that. It's in the process of being fixed.
"Yet, in some places where the signal was relatively weak, the iPhone 4 showed no bars, or fewer bars than its predecessor. Apple says that this is a bug it plans to fix, and that it has to do with the way the bars are presented, not the actual ability to make a call. And, in fact, in nearly all of these cases, the iPhone 4 was able to place calls despite the lack of bars."
Full review here:
http://ptech.allthingsd.com/20100622/apple-iphone4-review/
Thanks for the info but it looks like Youtube is blocked in my office!