I have a Motorola Droid X and I noticed on the display what appeared to be a small oval of moisture behind the glass or at least so I thought. I had no idea how it got there, but I was thinking I would need to leave the phone in the sun for a while or try using a hair dryer on it to get the moisture out. I tried this a little, but nothing too extreme, because I didn't want to harm the electronics. I showed this to a couple people and they had no idea what the problem or solution might be. But then by chance I noticed that when I removed the battery, the oval went away! It turns out if you press the panel that the battery rests against you can cause these ovals to appear. My battery is over two years old and it was the battery itself causing the problem, because it was beginning to bulge slightly! Now that I think about it I remember this happened to me also on an old Motorola flip phone, where the battery cover would no longer stay on properly and that too was caused by a bulging battery.
But now I've upgraded to a Droid RAZR HD, so only discovered the problem too late. But hopefully this post well help others if you encounter the problem!