I will have a verdict for you on this shortly. The reason for this is that up until now I would have said no, you should not upgrade because the proximity sensor issue has not been fixed, and I've not experienced a single call on my iPhone 4 which did not involve hanging up on someone, putting them on hold or mute, conferencing in a 3rd party, deleting an e-mail, etc. However, I just now realized that the 4.1 iOS is out, and that I don't have it, and that supposedly this resolves the problem. We'll see, and I'll get back to you. If it does fix the issue, then I'd definitely say yes, it's worth the upgrade, and if not, I'd definitely recommend waiting for the iPhone 5.
I bet you could have upgraded to 4.1 in the time it took to type that. But you might have hung up on someone in the process. (4.2 will be out soon also, it's on beta3 already.)
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So anyway, I'm now on 4.1. If you want to accelerate the testing process, give me a call.