No, a few hours of the Wii being left on will cause no damage at all. On my Samsung 42" Plasma I left my Wii on overnight (on accident) and left in the morning for class without even noticing it in the other room. I got home at noon, and the TV had the Wii menu image on it for a total of 12 hours and there was no burn in at all. Not even temporary.
As far as the bars being "bright" grey, they are certainly grey instead of black, but they are hardly bright. I watch movies on my Plasma quite often and I would never notice that they are grey unless I turned the TV off from that screen (seeing the contrast between that and black). I'm not sure what type of Plasmas you used for your presentation, but even a medium quality Plasma shouldn't have experienced that issue. I'm not going to stress test my Plasma for days or anything but it's not affected by being left on for any reasonable amount of hours even with a static image like a pause menu. It's very true that this was an issue with early Plasmas, but no recent plasmas of any decent quality will experience burn-in under reasonable circumstances.
Samsung has some of the best TVs in the market. I don't mean anything by that beyond the fact that you were smart in your purchase, and that your TV is definitely above average, even if you payed bellow average. But believe me, most other plasmas (and flat screen TVs in general) don't behave nearly as well as the Samsung ones.
As to the gray bars, I was talking about 4:3 content not movies - don't know if you usually watch that. LG plasmas (for example) put horribly bright gray bars/pillars on 4:3 content. And by horribly bright, I mean 50% gray, and no way of mistaking it for black. Yes, you can turn that feature off, but you'll get permanent damage in months if what you do the most is watch 4:3 content (as I do). On movies, 2.35:1 bars are not usually introduced by the TV, so they'll usually be true black, as black as the TV can do (which is definitely a problem for some plasmas, if all you do is watch 2.35:1 movies).
In my experience, burn in is still a problem that can't simply be dismissed with "no good plasma will burn in on reasonable conditions". Leaving your Wii on for a week isn't reasonable, but watching a majority of 4:3 and 2.35:1 content is. And it's something that people buying plasmas should be aware of, specially given that the problem is most acute on the first 100 or so hours of the TV's life.
OTOH, my experience with fixed pixel displays (plasmas and LCDs) tells me there's really no much difference on jaggies, except maybe that plasmas consistently have less horizontal pixels than LCDs, so maybe they'll display anamorphic content a "Wii bit" better. It's got much more to do with the quality of image processing than anything else. There's always that argument that 1080p is worse for SD content than 720p because the scaler has to fill even more pixels, and quite honestly IMO that's only ever a problem if your 1080p TV scaler sucks more than a 720p TV scaler (which it shouldn't).
So, really, YMMV - and all that matters is that you're happy with your own TV.