It's the first question we get whenever a "world phone" like the Motorola Photon 4G is released. "Can I stick my AT&T or T-Mobile SIM in it?" By all means, go ahead. Won't hurt anything, so long as you're in the U.S.
And it also won't do anything.
While there's hardly a large number of people who would realistically buy something like the Motorola Photon 4G on Sprint (or the Droid Pro or Droid 3 on Verizon) just to use it on AT&T or T-Mobile -- it'd still be pretty cool. But just like we found out with the Droid Pro (and other world phones since), Sprint and Verizon have locked down their world phones so they won't work on T-Mobile and AT&T in the United States. And we're not just talking about needing an unlock code here (again, see the Droid Pro). Until someone hacks their way into it, these things are DOA on GSM in the USA.