This is being discussed in a couple different threads, so I thought it most efficient to start a new one rather than repost in several places.
Got your new Pre 3 and not getting 4G? If you had an older 3G phone you have to call AT&T and get them to change your provisioning to allow 4G. The icon next to your signal meter should be an H+ for faux gee.
I called. The CSR didn't care that it wasn't an "approved" phone or none of that. He wanted the IMEI number (available under the battery or in Device Info->More info), spent a few minutes reconfiguring and that was that. Faux Gee!
I had a few extra problems because I am under several grandfathered plans. The unlimited data was no problem for him, but my 200 SMS plan caused him to have to escalate a request for reinstatement upstairs, he said it would be no problem, as I was grandfathered into that plan, just that he couldn't do it at his workstation.
I am not seeing exceptionally high speeds, but higher than 3G and it works through my microcell just fine. No changes needed to the configuration of the microcell.
Make the call.... have 4G.