It’s not good news. I had my faulty initial iPhone 4 replaced. But the replacement wasn’t much better. Signal strength seemed much better and I didn’t drop any calls, but the proximity issues remained. So they replaced it again. Now I am on my third iPhone 4.
Unfortunately, proximity sensor…
I think it’s partly software — I upgraded my iPhone 3G to iOS4, and I’m experiencing some odd behavior as well occasionally (i.e. sound source switching from regular to speaker).