Why is everyone encouraging the use of P2Pool as the answer to the 51% problem?
BitPenny seems to be half the solution - it has centralized control, but its clients can verify it behaves nicely in some sense. But it still has central management. A 51% pool can pull off subtle attacks that aren't prevented by BitPenny. For example, it can prune competitors' blocks, which is not always detectable due to the network latency. It can also block certain transactions or all of them. See this Hacking Distributed post for more details. BitPenny won't always detect this behavior. 

P2pool is the full solution - it is a truly decentralized mining pool with no central control. There is no entity that can wield its power to do anything other than honest mining.