I had this happen on my HTC Touch (winmo) also. The touch screen is probably de-laminating from the LCD. It will function fine but it will get worse, if you can easily see it take it back, that is a manufacturing issue. When I took my touch in to Sprint they swore they couldn't see what was wrong but they changed the phone out anyways, Good Luck!
This has been reported for a while. I now have 2 white blobs, as you put it, on the right side of the screen and now a few on the bottom of the screen. I am in the same dilemma, do I just keep the phone or exchange it. I have had this since day one with no problems except for the white blotches. How do we know that it is not going to sooner or later happen on all the EVO's. It could be a hardware problem with HTC devices. I don't want to go through what I went through with the Pre. It took 5 phones to get a good one. If I stop by a Sprint store, maybe I will have them look at it and see what they say.