Eligius does it too right? They don't guarantee it 100% of the time but most payments are made that way.
EDIT: To clarify how this works, the miners who contribute to mining a block don't necessarily get paid from that block, their shares get credited like any other pool and upon reaching a minimum payment threshold their eventual payments are made using some future block's coinbase. That avoids the issues p2pool has with dust payouts vs. minimum share value.
Right - that works for big pools that find blocks often. If you aren't a huge pool then payment variance (or just payment waiting period) would not make miners happy :p
Latest code should be working very well with vardiff and share acceptance.
Payment processing and IP banning to mitigate low-diff share attacks is under development. Right now I'm stuck on getting address validation. Looks like it should be: slice off 4 byte checksum -> decode rest as varint -> keccak (w/ 64 byte output ??) -> compare result to checksum.