It will be on September 29, 2013 at Wembley Stadium with kickoff at 1 p.m. EDT (though given the time difference, it'll actually be 6 p.m. local time in London).
The Minnesota Vikings will be our opponent, and perhaps more importantly - they will be giving up a home game for it. It's not too big a surprise that the Vikings would agree to it (given that their new stadium won't be done before then), but it's nonetheless reassuring to know that a fanbase that always shows up for home games won't have to miss out on one.
"We are excited for the opportunity to play in London in 2013 against the Minnesota Vikings," said Steelers President Art Rooney II. "It will be an excellent chance to grow the game of (American) football internationally, and we are thrilled that our players and Steelers fans around the world will be able to enjoy the atmosphere. We look forward to playing in historic Wembley Stadium in what we are sure will be an entertaining game for NFL fans everywhere."
(I added the "(American)" to the quote, just so you know that we know that "football" does usually refer to a different sport outside of the U.S.)
It's not quite the game in Ireland that Steelers chairman emeritus Dan Rooney had expressed interest in as the current ambassador to the Emerald Isle, so who knows if/when that'll happen.