[Thanks to everyone who sent this in]
Update: We've just spoken to AT&T and they've told us they're "aware of the reports." We'll keep our ear to the ground and update as soon as we have more.
Update 2: AT&T now tells us that it all came down to a "routing issue" in the Northeast that was patched up at 11:56AM this morning. For what it's worth, BlackBerry and text services were apparently unaffected -- but we doubt that's much consolation to iPhone users in withdrawl.