There are various ways to handle session timeouts, like \"meta refreshes\" javascript on load functions etc.
I would like something neat like: 5 minutes before timeo
Use some jquery that keys off of your session timeout variable in the web.config. You can use this Jquery delay trick that when a specific time occurs (x number of minutes after load of the page), it pops up a div stating session timeout in x minutes. Nice, clean and pretty simple.
Regarding session timeout, Codesleuth's ajax call would be perfect.