As an ASP.Net developer using in-proc session you have probably run up against the issue of session loss. This usually happens due the application pool restarting. This happens for various reasons, usually due to a file or folder being changed or deleted in the application folder.
Well, we had finally resolved all the programmatic reasons for this and had on customer who was still having issues. After RPCing into his server and looking around I noticed that Google Desktop was installed on the server. No one knew why. However, after uninstalling Google Desktop all the issues went away.
So, if you are having session loss with in-proc sessions and you have Google Desktop either remove it or set it to exclude the ASP.Net application folders.