If you're trying to access your session data from outside the regular php session functions, you might want to use WDDX as your serializer, as opposed to the normal php serializer. When your data is serialized as XML, obviously it's easy to unserialize as you please.
WDDX seems to be a little slower, and the text string it creates is much bigger than that created by the normal php serializer, but it provides the functionality with minimal hassle.
Donal