I got the exactly same problem too, but I have a little guess about the reason for occuring it on my machine...
I have installed the new Internet Explorer 7.0 Beta to test it out and to develop apps kind of "future proof"...
As i want to have my webapps also be displayed correctly on Internet Explorer 6, i installed the two IE's in a way to have them working parallel on the same machine...
I think that this is the reason for my troubles because when i use Internet Explorer 7.0 to view my site, it works perfectly.
When i use the Internet Explorer 6.0 it fails with the "ASP.NET session has expired" error.
Now when i try to open the same page on another machine, where Internet Explorer 6.0 is the only IE installed, it works like it's meant to.
Is there a chance that you are using the same IE "parallel" configuration?