Awesome. A module like this has been needed for quite some time; Now people won't have to go through the hell that I went through :). I like a whole bunch of things about this module:

1. First off, I especially like like how you interface directly with the session file, rather then trying to interface with php itself by going through multiple layers (MySQL,Apache::Session,and external scripts). Interfacing directly with the session file gives a HUGE speed boast.
2. The functionality and Interface are top notch - especially when save is implemented.
3. I like the design of this module as well. Extreme modularity allows for easy subclassing if the user's system configuration is whacky. Keeping the (des|s)erialization functionality in a separate module allows for scaling if the php people change the serialization.

So, in preliminary review, PHP::Session looks to be a great module when its finished. Miyagawa++!