My proximity sensor issues went away with a software update that apple did. Once they updated the software, I never had an issue with my proximity sensor again. I did have an issue with my home button (it would not consistently recognize multiple clicks), and Apple replaced it (although I did have the extended warranty).
If I were you, I would wait until the iPhone 5 comes out. Also, make sure you get Apple's extended warranty. Any smartphone is going to have issues, and Apple has upheld their warranty very well in my experience. Totally worth it for me because I'm a heavy user and when something goes wrong with my phone, I just want a new one right away (can't wait to have it fixed!)