In case you're not subscribed to xorg-devel, I've sent off the first draft for the multitouch X protocol specification (as part of the X Input Extension 2.1) today. If you have interest in multitouch, please read through it and see if you can poke holes into the current approach.
Note that this is very low-level stuff. The data described by the protocol is what's sent on the wire to the client. Most applications are separated from this data by toolkits (GTK, Qt, etc.) and only few people are unlucky enough to have to care about the protocol itself.