http://forums.turbotax.com/intuit/board/message?board.id=5982F970&message.id=3660"An electronic postmark is issued the second you file the return electronically, regular postmark on the extension when you drop it in the mail. Both will set you as having filed on time. (Make sure you pay by the 17th, midnight, your time no matter how you do it.)
If you file electronically and get rejected, you have a few days to refile and get accepted or drop it in the mail and still be considered on time. If you drop it in the mail, include a printout of your status page that shows your electronic postmark."
So the timestamp that you Submit the return (not return to see if it is accepted) is the virtual postmark. As long as you submit by the deadline, you're fine.