Session expiration

I recently moved to memcache for session management and I am very happy with the performance.

However, I need a way to trigger methods when a user is logged out, including when the session timer expires. I can think of a few ways to do this when using file and ActiveRecord sessions, but is there a way to do this with memcache, or better yet a method that is not dependent on the session store.

Any ideas?

Thanks, Jon