I had this problem crop up several months ago on an EVO I bought at launch. After some googling, I discovered it's commonly referred to as the "B spot," since the most common location is near the B on the stock keyboard. Took it back to a Sprint store with repair center prepared to argue about it, but they took one look and replaced the screen for me (gave me a screen protector to boot). So, if you haven't already, take it to a Sprint store (preferably one with a repair center).
Do you have insurance? The reason I ask is I just bought a used evo on craigslist... I met the seller at the sprint store and I didn't notice the white spot until after the sprint rep was activating the phone. He claimed it was normal. I did have something similar on a palm pre in the past. I don't have insurance (since I own 6 phones) but I've heard from others if you bring the phone to sprint they will repair it for $35 without insurance. Can anyone confirm this? Thanks.
UPDATE:
So I went to the Sprint store and they fixed my phone! I just bought this phone used and it had some issues that I was prepared to deal with but now I don't have to

The phone had the "b-spot" as well as another bright white spot in the center. The earpiece speaker was crackling and the grill was discolored, the capacitive buttons didn't light up and the adhesive on the underside of the screen was coming undone so it was visible, the power button was a little hard to press, etc. The phone had been dropped by the previous owner and also had a chunk missing out of the corner. I thought for sure they wouldn't want to fix it... even though the "b-spot" is a known issue, I figured they would claim the drop caused it. I asked the Manager (who happened to be helping me) what my options were to fix it since I didn't have insurance. She said $35. I was ready to pay and she asked if I wanted to add on insurance so I didn't have to pay! I said sign me up! I can't believe she even offered this as an option... this goes against the whole principle of insurance!! She even suggested that I could then go home and cancel the insurance on their website (which I did). I don't need insurance since I own several phones... I guess I can consider myself as self-insured! haha If I only had to pay $35 to fix a phone, that would sure beat $7/mo. If I lose a phone, oh well, I'll activate one of my spare phones rather than pay $7/mo and a $100 deductible! I guess the loophole is you can add insurance within the first 30days of adding a phone to your account. I never would've guessed this would apply for a used phone, but it does! Long story short, they replaced the entire front half of my phone for free! New glass, new LCD, new housing (bye bye chunk missing) new speaker and grill, new power button, etc. I would've happily paid the $35 but if it's free, it's for me
UPDATE 2
I got home and my SMS wasn't working. I kept getting an error and the few texts that I did receive were being delivered multiple times. I spent 2hrs on the phone with Sprint. They updated it and provisioned it and did everything in their bag of tricks including a hard reset. Nothing worked. They were convinced that the tech messed it up when he replaced the screen and they told me to take it back to the store! I knew that couldn't have anything to do with it, but I went anyway. I was using my Evo a few days ago on Froyo and everything was fine with SMS. I activated my Epic and SMS was fine as well. When I got to the store this morning they made me activate the Evo before they would repair it. I hadn't activated my Evo since updating to Gingerbread a few days ago. This was the problem. Apparently there is a known issue with a small amount of Evo's after updating to Gingerbread, SMS gets messed up. The tech told me they have an update scheduled for 7/28. He didn't want me to wait so he gave me a NEW Evo on the spot for FREE!
