You could also try holding the phone a little lower. The proximity sensor is located right above the ear speaker. There's an LED there that emits a light, and the sensor detects whether the light is reflected back or not. If you don't sufficiently cover the sensor, the light never gets reflected back and it doesn't trip the sensor. Holding the phone a little lower might do the trick.
Try this, while on a phone call, move your finger around the area above the ear speaker until the screen turns off. If it doesn't turn off and STAY off, you probably have a bad sensor/LED. Take it back to Apple Store and tell them the problem and they should replace it for you.
09-22-2010 12:19 AM