I REALLY would like to know the answer, because I'm suffering from the same since two weeks and it is a pain ...
Session variables keep corrupting to "resource id#2" or so (value returned as string : "2").
For instance, if my php script is in a frame, I have corruption, if not then it's more rare...
That's why I was on the verge of checking bug status for PHP 4.0.0.16 and Apache 1.3.12, considering to upgrade if necessary ;-)
Good luck and please keep me informed if you get an answer why 8-)
Vincent