I don't know about P2Pool, but on the bitcoipool.com FAQ, they have this to say...

Q: My hashing speed (mhash/sec) is different on the server and on my miner?

We view all the shares that were submitted in a 5 minute window (from 5 minutes ago to now), multiply by 2^32 (the size of a getwork) and then divide by the number of seconds in 5 minutes. Then divide by 1000 three times to make Gh/s. It's fairly accurate. Obviously you'll see inflated speeds if you submit more than one share per requested getwork, and the reverse is true as well. It's estimated. As there is no way for us to query the client itself, we have to guess, and this is the closest way to guess.

I've noticed that my hash speed on the pool is ussualy about half of what my cg or bfg miner says it is running at.