t-mobile have a slightly different way to operating their HSDPA network (incidentally, O2's is about the most dimuinitive hsdpa network in the world)
the way t-mobile s network is setup, the phone will display 3g coverage whenever it has it. only once data is actually transferred on the network will t-mobile up the data transfer speed and switch you to a hsdpa connection. this allows a considerably larger number of users to be connected to a mast at once, and lets more devices transfer data at the faster rate because devices not transferring data will not be connected to the faster network
i have only ever had hsdpa when in a 3g network, at it generally performs at aroumod 900KBps-1MBps (this is with a HTC TyTN II)
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