All plasmas, by their nature, will get some degree of image retention if a still image is left on the screen for an extended period. Burn-in is permanent, but newer plasmas are less prone to it than older plasmas. Owning a plasma does require some bit of maintenance just to make sure burn-in doesn't occur (especially if you are using it as a PC monitor).
Since computers typically have much more static images, with icons/windows/borders etc usually staying in one place, plasma tv's are not ideal for computer use. Plasmas, after all, were designed for dynamic motion (television, sports, games) as opposed to LCD/LED which started as computer monitors.
Most plasma tv's will have a burn-in utility, which you'll want to leave running if you do have some burn-in. Using the Mini, you do get black curtains which on a plasma will cause image retention (or if abused, burn-in). It has nothing to do with the Mini itself, simply the static image on the screen. It's important to take breaks, whether using the computer, Mini, or whatever. Watch something else every few hours or turn on the burn-in utility.