Long story short, I played too much videogames and the health bar of the game burned itself in.
What would you do? It's very faint, but worrying.
Most plasma TVs have features that prevent and/or erase burn in. In most cases a scrolling white bar or static noise will erase the burn in. Don't give up right away. Read your TV directions and give it a go. Burn in is much less a problem with the newer plasma TVs.
I have the same issue with mine. I will typically hold down my PS button after every 15 minutes if there is something constantly on the screen that is white. If you see some burn in, turn the tv off for a few minutes, it should fix that.
I have a 22" LCD monitor already (Which I use for PS3 and PSP) but I use the console on the 50" Plasma sometimes. It's just that I got carried away.