The announcement was made in advance of the annual conference sponsored by CTIA - The Wireless Association. AT&T Mobility CEO Ralph de la Vega is scheduled to give a keynote at the conference tomororw and could use the event to further elaborate on their intentions.
The announcement is most likely a response to recent events at the Federal Communications Commission (FCC), which is proposing new rules that would prohibit carriers from blocking apps on their networks.
A public announcement by AT&T stating that they will allow apps such as Skype on their network would remove any doubt that it's the company that is doing the blocking, and put the spotlight firmly on Apple to explain why they're not approving VoIP apps. As noted yesterday, VoIP provider Vonage has made its app available in the App Store, although ratings after the first day probably make them wish they hadn't...
It should be noted that the announcement will likely have no bearing on the fate of Google Voice, which was rejected by Apple on the grounds that it duplicates functionality already found in the iPhone. The Google Voice app allows users to control their Google Voice account, send SMS messages and check voicemails, but does not provide any VoIP functionality.