Thanks guys, you've convinced me that burn in isn't anything to worry about for new plasmas. Worst case, a few minutes of image retention that goes away by running the scrolling bars, or just watching something else is a minor inconvenience.
That said, I was at Best Buy on Sunday comparing sets, and am going with the Sony HX929.

In their home theater section they had the HX929 and a GT30. That area was darker than my living room (which has multiple windows), and I still appreciated the extra brightness and AMAZING picture of the sony.
PS - comparing screens at BB is a real pain in the ass. Most of the time, the screens are playing video that changes very quickly - so by the time you turn your head to look at another screen, it's a different image already. Then they showed some clip about Pearl Jam that looked like it was shot on grainy 8mm film...ugh. Then, finally, the content switches to a national geographic special on colorful and (slow moving!) seahorses. And some dude with serious BO stands right in front of me... Sorry to vent, but looking at big screen tvs required way more patience and was much less fun than I thought it would be.