FYI:
We use a WebSphere Edge Server (Network Dispatcher component) and clustered Domino servers that use session based Single Sign On.
Web users were intermittently having to sign-in twice (especially when using Windows XP) because they were being load balanced to another Domino server before that server was aware they were already authenticated.
We solved this be enabling the sticky feature on the WebSphere Edge Server.
Hope this helps someone.
-Jason