How do I find the hash "market share" of P2Pool?
In concept, P2Pool is much stronger than centralized pools. Unfortunately, most centralized pools still hold a high hash-rate.

You can see the global hash rate, and its distribution at http://bitcoinwatch.com/ 

My opinion is that miners are used to using slush, deep bit, and btcguild - and are either unaware of the incentives of P2Pool, or don't care.