The only way I've seen to keep the net connected when you're not using it is to use it.

If you for example sit there and ping a site or leave Pidgin running then it should stay connected indefinitely, as soon as you stop using it for a while it disconnects.

This is a good thing by the way because as I can recall the Wifi bug that supposedly leaves Wifi on when it's supposed to be turned off eats about 150 (or more) miliamps per hour, with a ~1200mah battery you're not looking at very long life if you just leave it on all the time.

At idle, I've noticed the n900 with no net connected to use about 5-10 miliamps per hour. (Which means your phone will last a lot longer on a single charge when you're not using it.)