Smart move. Given that they are getting increasing customer complaints about poor service and coverage for their 3G network driven by cusrtomers actually wanting to use the data service it makes sense for them to conceed the voice business to the data side of the network to keep people happy.
The truth is that voice will not create huge congestion on the data network since full duplex VoIP is 90kbps or so. This is nothing compared to the multi-meg video apps people want to run. It will allow for usurping of the traditional voice network and then therefore revenues to AT&T. This is inevitable.
Allowing for this now will help to keep customers sticky when the exclusive deal with Apple expires. The more people use the device on the AT&T network, the more their passwords are pre-programmed and their contacts database grows, etc the less likely they are to leave. If AT&T did not allow VoIP on the data network it would have only accelerated the exodus from the AT&T service.
Once we get to IPv6 End User ENUM might actually happen after all, but it won't be ENUM, it will be SRV!