The outage, which appears to have begun early this morning seems to be affecting both 3G and EDGE connections. Voice and SMS communication is unaffected. Users have been reporting the issue from as far west as Missouri and Illinois, as far north as New Hampshire, and as far south as Virginia.
There have also been some unconfirmed reports that indicate that the outage is only affecting iPhone users. (Update: The outage is not specific to the iPhone).
Update: Service has been restored to many users.