I too have been looking for something like a "top posts" plugin. So I've hacked on this idea a little bit, and put together a simple prototype of "top posts"... you can find the first cut at it here...
http://heftagaub.wordpress.com/2007/03/11/wpmu-top-posts-plugin/
The page doesn't contain a lot of documentation yet, but the code has more than enough.
This is very early, I'm sure it could be optimized, I haven't tested it that much but it seems to be working in my test environment.
I tried to make it work like get_posts() and I also added some template style functions... like get_the_title() and the_title().
I don't know if there is a better version of this out there, but for now, here's a cut at what I was thinking.
Not sure if there is a better thread to post this at, but for now I posted it here and on another thread where I saw this concept of "top posts" discussed.