Is fast user switching in Lion disabling the network connection for the accounts that aren't active? With 10.6, I routinely had two accounts logged into the Mac while I was working - my personal and my work account. I switched between them as needed and this worked fine. For example, if I had an IM client running under the work user and switched to my personal user and then back again to the work user, everything was ok. The work user's IM client stayed connected the whole time.

With 10.7, it looks like switching to another account renders the network connection inactive for the account that is no longer in use. When I switch back to the work account now, the IM client has to reconnect and the VPN attempts to reconnect.

Is this new behavior documented somewhere and is it configurable?
Not really - Lion is now able to kill processes that appear to be idle and unused. Fast user switching adds more load on memory and processor (due to the simple fact that more is now running), increasing the chance this will trigger.

Also, the changes to allow multiple simultaneous logins may have changed the API which will need the VPN client to request to stay connected or otherwise start itself outside the user space to survive a switch in Lion.