I have a c8000 plasma, and it gets IR very quick. I'd be very nervous about using it as a monitor. You'd probably have the start button permanently burned in before too long.
Image retention and burn-in are diffrerent things and happen through different processes. With image retention a static charge builds up in the display elements, with burn-in the affected display elements just wear-out faster (or less fast) than the rest. Image retention doesn't take long to happen, but isn't permanent. Burn-in takes much, much, longer to happen, and it's permanent.
Burn-in can't happen too quickly because it's the result of uneven wear. The display elements across the entire panel will all slowly wear out with use over time. If this process happened quickly then it wouldn't be long before the entire screen was too dim to see.
I'm not sure how long it takes on current plasmas for actual permanent burn-in to occur, but it doesn't seem to be much worse than with CRT TVs, and more relevenatly CRT monitors. My CRT monitor has a slight bit of burn in at the bottom where the Windows taskbar sits, but that was after almost 10 years of use and I had to look for it to see it.
Plasma TVs have a lot of potential advantages when used as computer monitor, but I'd only use one primarily that way if I could accept, say, a five year lifetime. That's assuming the temporary image-retention problem doesn't become an immediate annoyance.