Ye, for both 3G and WiFi.

If you have not set up any push app, the connection will auto disconnect.
But then, I wud like to warn you that since PR 1.3 update, auto disconnect doesn't work properly.

With clean device, it worked for few days, but as I went on installing apps and setting up push services, the auto disconnect wud work only once after toggling net connection each time.

Ofcrse you can turn on power saver mode, then it always auto disconnects faithfully. Not sure if there's a tweak to control brightness manually in psm, then I won't mind keeping psm on always.