Burn-in is for all practical purposes a non-issue with current plasma HDTVs. HDGURU.com did run some burn-in tests but came to the conclusion that is a non-issue for all current plasma TVs.

Attempts by the HD Guru to create “burn-in” (uneven phosphor wear) on 2008 and 2009 model year Plasma TVs by freeze-framing an image for 10 hours and then switching to a white screen have been unsuccessful.

It only occured after a number of days, when a full white 100 IRE square was put in the center of the screen and the user mode was set to Vivid and the contrast maxed out. This is not a "real-life situation" and no consumer will encounter this, even if you would simulate this a screensaver which is activated by default on most Plasma TVs will prevent this.

You can read the complete test here.