I'm having the same issues and it's really annoying! I've accidentily conference-called 3 people today, hit the mute button at least 4 times and pressed a couple of numbers by accident! Argh!
The thing is, I have always been annoyed by the screen no matter the version -2G, 3Gs, now iPhone 4. I realize that while talking and simultaneously doing other stuff, sometimes you just shift and the proximity sensor turns off, so what I used to do was turn off the iPhone screen by clicking the sleep/wake button once, while I was on a call (this did not end the call). Then to access the phone again, all I would have to do is click once again and it would bring up the "slide to unlock" screen.
I am now under the impression that this "feature" was only available in software version 3.1.2 (not sure if it was possible for versions before that, but I know the next version up you're not able to do this - my friend has a 3G running 3.1.3, and wasn't able to do this). Now when I hit the sleep/wake, it automatically ends the call 🙁
If Apple just let us do what I could do in 3.1.2, that would be great!!! Has anyone else been able to turn off the screen while on a call by clicking the sleep/wake button once?