IPhone 4 – bad proximity sensor

I think the proximity sensor in my iPhone 4 is out of order.

Nearly every time when I use the phone to place/receive calls, suddenly the mic mutes itself, or the speakerphone turns on, or a call starts on the second line — but I am not doing this! I am merely holding the phone up to my ear (in a quite normal way). This was never a problem on my iPhone 3G.

It seems to me that the proximity sensor isn't sensitive enough and turns on the touch screen sometimes. I've heard that there used to be problems with that back on iOS 3, but I've been running 4.1 since it came out and the problem remains.

• Is there any way to check if my suspicion is right?
• Can I do something to fix this myself? Is there a fix in iOS 4.2.1?
• Do I have to go to an Apple shop and wait phonelessly for weeks while they send it to the factory?
  Note: I'm in Austria, we don't have Apple Stores.

I feel forced to use the headset in order to avoid these problems. While that has some advantages, it's not always practical, and anyway I want this fixed. Here is just one of the stories about this; so I think I'm not alone.

<rant>It's a #¤%! expensive phone and it better work…. I'm asking here rather than the Apple support site though because that other site has so few answers but page after page full of screaming people.