I just did a search for posts looking for info and pics about people painting their interior trim pieces and didn't find any but this one. I think that is pretty strange, but whatever. I've been researching painting plastics and I'm pretty convinced I can do a decent job with the interior trim pieces, but still not sure if I should use primer, but I guess it wouldn't hurt.
To experiment, I painted my cell-phone cover. It came out pretty well, (Duplicolor paint) however there is a small button on the back, used to take the cover off, that is a different type of plastic. The paint Immediately chipped off of every millimeter of it. Clean off. However, the rest of the phone is quite durable, and I haven't even gotten a ding.

I wet sanded the clear-coat layer (1200) and then used some rubbing compound to buff it out. The result is a very smooth "deep" finish.
I don't have a digital camera, or I'd post some pics of the finish.

I would definately recommend using a clear coat if you want a nice and shiny finish.

Most pages I've read about auto painting recommend wetsanding with 400 then cleaning with a laquer thinner.

Side note, I now get to wax my phone for extra shine. :thumbup: