In fact, I think Apple was a little short sighted for not putting in a dual-mode chip and making a single unified model that worked on both carriers. But, I bet AT&T and Verizon would rather they be separate so they can lock you in to one US carrier.
Makes no difference to the carriers whether it's unified or not, although I can see Verizon benefitting from current arrangement more since whoever buys the CDMA version is locked to Verizon. From consumers' perspective, an unified version will allow them to be more flexible with their telco choices. In fact now that iPad 2 supports quad band including GSM 900mhz I think it will run on T-mobile's 3G as well? Correct me if I'm wrong.
Edit: oops. No it won't work on T-Mobile's.