So it looks like the sketch cover (pictured below) was not released. I'm not sure where the image originated, but a few auctions made it onto Ebay before the convention began. Oops.
There was another giveaway with a Spidey appearance at the con though. Marvel gave away a preview version of Amazing Spider-Man #692 that also includes previews of Hawkeye #1, Captain Marvel #1 and Gambit #1.
I first reported there were 2 variants at the 2012 San Diego Comic Con with Spider-Man making an appearance, but there are actually 3 showing up.
Amazing Spider-Man #688 (regular cover)
Amazing Spider-Man #688 (sketch cover)
Astonishing X-Men #51 (with the blink and you'll miss it Spider-Man glove)
And if you can't make it to the show, you can find them on Ebay.