If you mean, without a long contract then it depends: AT&T will not let you use an iPhone with their prepay accounts (which I didn't know when I bought my iPhone).
If you mean, just buy them and use them otherwise without any phone ability, yes.
They don't let you... but there are ways to do it.
I'm in Canada and use an unlocked AT&T iPhone on Roger's network without data, had no problem, they even were the ones to install the SIM in my phone. I also had put my SIM from my payphone in the iPhone without any problem before that.
If you are using an iPhone that's not locked to another carrier, how would they know/care what phone you put their SIM into? I guess you'd be beter off giving them a call.