Carrier StraightTalk + iPhone Upgrade Program

So I'm on StraightTalk (AT&T), but wanted to do the iPhone Upgrade Program on launch day. Technically, a condition of the plan is activation on AT&T, Sprint, T-Mobile or Verizon. Hoping for the best, I reserved an AT&T iPhone. Yesterday, I went to the local Apple Store and picked it up. I had to sign up for an AT&T plan, but I wasn't required to activate/unbox in the store. At home, before powering the iPhone on for the first time, I removed the included AT&T SIM and put in my ST SIM. I powered it on and everything works great. Since it was never even on AT&T, I can't cancel the plan. I called AT&T today and asked them to cancel my AT&T phone number (printed on my Apple Store receipt), but they said there was nothing to cancel. My assumption is that if a certain number of days goes by without activation, the plan is cancelled. Since the phone is financed through Apple, not AT&T, I don't think this should cause any issue. I guess I'll know for sure in a few weeks.

But from where I'm sitting right now, this seems like an easy way to buy an unlocked iPhone and use it on ST, or whatever provider / MVNO you want... TODAY.