All of this "open network" stuff is just hype. GSM by definition is open...you can bring any GSM phone to any GSM network, you just need the SIM card. The one exception is of course the iPhone, but even that is just a software block.
AT&T is just spinning this in reaction to Verizon's "plans" to open their network, which still leaves the devil in the details.
Read Dieter's take on it, which I agree with 100%:
ATT to Verizon: You Want to be Open? You Can't Handle The Open!