I'm running this on 3.1 without any problem whatsoever.
Right now, I'm loving it's behavior, BUT...on a previous theme it was an absolute nightmare in where it would place itself (i.e., flush against my AddThis bar).
My own view is that this is worth a try for most bloggers - I gave it the four stars - but many will find the inability to suppress it on certain categories and specific posts is a real problem.
On some themes, the inability to customize padding and font size/color will also be a deal breaker.
That said, it does do what it promises, and the price is very right. :)