You can use plugins in WPMU, can't you? If so, here ya go gadsense.zip It's a widget to put adsense in your sidebar. You can actually plug in any ad code you'd like, it's not limited to just adsense.
Sure, it won't help if you want to place the big rectangles on your post pages or just below your header but, it's a start. It's easy to use and best of all, it's free.
Oh, and another thing I like about this plugin and completely forgot to mention...
It controls all the stuff in your sidebar(s) so you can rename stuff, move it around, take it out, swap stuff from side to side (if you have more than one sidebar,) etc.
I think it's pretty cool.