I personally wouldn't be too worried. So long as you're vigilant and don't, say, leave the Wii switched on at a menu screen for a long time, it shouldn't be a problem. Probably best to turn on the Wii's screen burn-in reducton feature. It really needs a stationary image for a long time to suffer screen burn in (eg channel logos).
Yes screen burn in is a problem with earlier plasmas, but if you're careful you should be ok. You could always get a small (26in?) LCD for playing the Wii, if you get concerned about your BV 5 suffering burn in. I'm planning on getting a BV 8 which will have my Wii connected, and as far as I know, LCD's dont suffer burn in, so that may be a better option.
Robert.