@pesos - you need to get your environment back into a supported state, that would be my advice. We're not going to be testing scenarios we don't currently support. I think it's a bit early to get into specifics on how we will be helping 2007 customers move to ABP's, but you should be prepared for some additional work to get you back to a known state. Sorry.
@Fung and Luis - Watch out for the SP2 information we will be publishing soon. As we said in the post though, using ABP's to try and host Exchange is not the goal of the feature, it will not compare to the true tenanting capabilities available when Exchange is used in /hosting mode.
@Liam - no, there will not be any client side requirements.