Right now mod_python comes with DbmSession and MemorySession. DbmSession performance can be pretty crappy if for any reason the anydbm module reverts to the dumbdbm implementation. With the current filesystems, implementing a file-based session manager should be easy and should perform well.
See the following thread :
http://www.modpython.org/pipermail/mod_python/2005-April/017825.html
I've checked in a preliminary implementation based on dharana's work. We'll have to cope with a few issues (namely locking) before it is final.