There's been some discussion on meta recently about accepted answer rates. I've added two new graphs to my stackoverflow graphs page:
The first shows how the rate of questions that have accepted answers. Interestingly, the rate took about a 10% drop early on (after SO private beta) and never quite recovered to the same level. The recent steep drop (on the right hand side) shows that the newest questions just haven't had an answer accepted yet.
The second is a user-centric view which shows how many users have each accept percentage accept rate. I used the same criteria as SO itself (more than four questions asked) otherwise the data gets even less interesting.