The default setting for all WiFi connections is the most aggressive power management. This means that the WiFi module will power down as soon as you're not sending data. This includes not receiving attempts to connect to the phone.

To actually stop the WiFi module from sending any data, you need to get down on driver level. libiw lets you do this. As long as it's not completely turned off, it'll continue to check if you've moved into the reach of any defined WiFi connection.

So, the actual gain in disconnecting from whatever WiFi AP you're connected to seems almost nonexistent as long as you use power management. But I'm sure people have done the measuring to get the data.

I can't really comment on 3G data connections since I haven't inspected that (yet).

And yes, the N900 is a Super Computer.