Be aware also of what setting your "wifi Sleep Policy" is set at. By default, the phones sleeps its wifi when the phone goes in to standby. In this case all your widgets and feeds and stuff are pulling data from 3g, which uses more battery and obviously eats away at your dadt quota (AT&T conspiricy?

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To change that go to the wifi settings screen, tap the menu button, tap advanced and change the wifi sleep policy to never. There really is no reason to be pulling data via 3g when you are on wifi, even in standby. That might not answer your origional question (which I feel was answered above already) but this may be of help as you try to get your wifi settings and stuff figured out.