We have many small Classic ASP applications now running on IIS7.5 / Windows 2008 R2.
Since we completed our migration from IIS5 we encountered some inconsistent behaviour around Classic ASP and Session State. It appeared that the session would intermittently be lost and then re-appear when re-submitting pages etc.
We eventually discovered that this issue was being caused by the Application Pool being configured to function as a Web Garden i.e. Maximum Worker Processes was set to > 1. When we reset the number of processes to 1 the Session State is now working as expected.
The same must apply to IIS6 as per the article which pointed us towards the solution: IIS 6.0 Session State May Be Lost If You Use Web Gardens with ASP Applications