Recall the al-Qaeda courier captured with a CD-ROM in Iraq? The one that contained the letter from Zarqawi, complaining about how the Americans weren't going away and that he was having difficulty recruiting?
While it didn't begin with "Dear Osama", the "you" that the letter was addressed to was obviously well above Zarqawi's pay grade. From context, it seemed evident that the "you" was more likely Osama than Zawahiri.
Accordingly, we can conclude that he remains in contact via courier. Doubtless, one known courier has been allowed safe passage and led our forces to Osama's lair.