Check the bug reports on php.net and see if anything there matches what you are doing. Lots of people are having session problems since 4.1.2 but its important to see how things are setup.
Personally I cannot get the session functions to work under a virtual host on an apache box.. they work in the primary site, just not in a virtual hosted site on the same system.. incredibly frustrating...
Lance
lance(a)metaldog.co.za