I've done a lot of complaining recently about handoffs between EDGE / UMA ... but between EDGE / WiFi when tethering I've had some decent luck I'd say...
So I fired up a constant ping (I'm at the car dealership getting my wife's car serviced) ...
First I'm tethered via EDGE, then I enable WiFi (profile already setup), and then it switches over.
Not too bad ... only one dropped ping, and a total hand-off / negotiation in a few seconds with me not having to drop the dial-up connection.
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