I just drove back from Vegas to Southern California this morning and aside from the few points that are EDGE and GPRS on that drive the Inspire showed H+ the whole way.
I ran speed tests the whole drive with the Inspire and my wife's iPhone 4 sitting side by side. (don't worry, I wasn't the one driving) There was little to no difference between the two the whole drive, telling me that if there is in fact "back haul" to be turned on it is not on along this route.
On a side note, AT&T is down right atrocious in Vegas. The speeds there were never even close to 1MB down and it took about 3 times dialing a number to get it to connect. Wow, I see why people hate on at&T so much! Where I live I have solid 3G coverage and always have about 2-2.5 MB down. The inspire had the exact same results at my house that my Nexus one has had for the past year and a half. I guess the "4G" is not on here either.