People over in the announcement thread for bytecoin have found out that GUIMiner just doesn't' work with p2pool (or bytecoin, but more likely the pool). If you are getting some authentication error and guiminer, it's not working.

Apparently, GUIMiner is incorrectly reporting shares or is doing something else p2pool doesn't like and you will not receive a payout (whether those shares are contributing is unclear at this point, I don't get any information from anyone using GUIMiner in my log files so I cannot confirm or deny any of this.)

[edit]
As of now, this pool donates the 2% fee to the faucet. Happy mining!