Surprisingly the N900 does alright there. Come home and it picks up the wifi signal quickly, go out and as far as I can tell there's not much difference in battery usage between leaving wifi in standby mode or manually switching it off. It does use power when it's connected though so sometimes I switch it off at home, and then somehow Nokia managed to make it mostly
work, but a couple of applications (the program manager/app updator and a weather applet) don't work over wifi without some extra fiddling. Apparently my router is at fault, but I've had the same with more than one router and nothing else has these problems. Oh, but try syncing against google calandar and you're somewhat out of luck: in theory maemo supports MS outlook sync or whatever which works at first, but after a week or two brakes due to a bug in maemo.