Ok, i switched to bitcoin 0.5, it seems OK now!
Yes, Bitcoin 0.5.0 is required now. I'll add that to the documentation.
but still get some "error like" messages:

Those are normal. There's an old peer mining a couple of shares.. I'll change P2Pool to make it obvious that these errors don't mean that something is wrong. EDIT: Fixed in
https://github.com/forrestv/p2pool/commit/59a9680