Not that it really matters much, but you are wrong.
In purely private (or internal) use with no sales and no distribution the software code may be modified and parts reused without requiring the source code to be released.
Hmm now the question is if a p2pool is private/internal or not.

GPL experts step forward plz.
Either way, it's good manners to release the modifications imo.
Sorry WOPR if my misleading post made you release it although you didn't want to.