Good idea, only a few issues... we probably wouldn't be able to give away tshirts because that'd get pretty expensive to do each month. And also:
Member Of The Month - Cool idea, but how would we choose? And based on what criteria?
Mod Of The Month - There's only 3 mods...
Car Of The Month - Wouldn't that kind of compete with rate-my-car?
New Member Of The Month - There are very few active new members, so we'd probably have a similar situation as with mods, with few choices.
Top Poster Of The Month - Would this be in quality or volume?