So I've had a Pre (now a Pre 2) and Touchstone since the beginning, and have been using 2.x for a long time on that. I've never had any issues with leaving Exhibition mode running on 2.x at all.
However, on my Touchpad, I've been using the Accuweather Exhibition mode most often recently. And my Touchpad is on the Touchstone for longer portions of the day than my phone is. So just now I swiped out of Exhibition mode, and my town name was temporarily burnt into the screen (especially visible on dark backgrounds). It's faded in the few minutes since I exited Exhibition, but it's still pretty concerning. My brightness is down most of the way, so it's not that causing it either.
Looks like I'll be setting Exhibition to something with more motion like the Facebook app or Photos. Not nearly as useful as seeing the weather at a glance, but considering this is most likely the last webOS device I'll ever own (not by choice), I'd rather play it safe.