For the record, here's the list of IDW comics with the three-page Locke & Key preview (all from January 2008):
Angel: After the Fall #3
Badger Saves the World #2
Cory Doctorow's Futuristic Tales of the Here and Now #4
Doctor Who #1 (preview also appeared in the 2nd printing)
Doctor Who Classics #2
Duostar Racers #1 (pretty sure it's there, but I have yet to see a copy)
Fallen Angel #23
Gene Simmons' Dominatrix #6
Gene Simmons' Zipper #3
Snaked #2
Speed Racer: Chronicles of the Racer #1
Star Trek Alien Spotlight: Borg
Star Trek: The Next Generation: Intelligence Gathering #1
Wormwood Gentleman Corpse: Calamari Rising #2
Eight of these had multiple covers (Angel, both Dr. Who, FA, SR, both ST, Wormwood)!