I have to agree with others prior, I don't think there will be two versions since there's enough physical space to commit both of the chipsets at once. Instead, actually, I think that Apple is talking with Verizon and asking them if they want to support this, giving them a fair shake at a slice of the pie, and then asking AT&T if they think this will sell if the iPhone crowd has to shell out yet another wireless monthly plan; in other words (me being all naive and what not..perhaps just hopeful), I have a feeling that Apple is going to be pushing AT&T to let iPhone customers piggyback their iTablet usage onto their iPhone data plans, maybe have non-AT&T users pay for a monthly plan and AT&T getting a percentage back for whatever gets sold on their network, same for Verizon.
This is, of course, purely speculative, but I really think Apple already knows that there are a ton of iPhone users out there who would be pretty upset if they found out their revolutionary new gizmo came with yet another $30/month premium to pay and will try their best to maximize their profits with the iSlate/Tablet/Pad.
AT&T likely knows this, but they're also aware that Verizon is quickly creeping into the picture, so most likely all parties will play nice since I'm sure Verizon wouldn't mind getting a cut of the Apple pie and AT&T wants to keep as much of it to themselves for as long as possible.