What "access point name" is defined under "Settings -> Internet Connections" for your TMO connection?

I live in a city with the new HSPDA+ service and I had the same experience. I realized, however, that I wasn't using the correct APN. I switched over to epc.tmobile.com and it tripled my connection speed.

In my case, I've often hit 8+ Mb/s and I'm loving it.