Stabby sneak 'em up Mark of the Ninja is indeed leaping from Xbox Live Arcade to PC via Steam on October 16, developer Klei Entertainment confirmed this morning. It'll cost $15 and no, just in case you were worried, it won't use Games for Windows Live.
"I'm primarily a PC person myself, and I'm not okay in any way with this feeling like a shoddy port," lead designer Nels Anderson told Rock, Paper, Shotgun. "It's my intention that this is a full, proper version of the game in its own right."
Mark of the Ninja launched on XBLA earlier this month to rave reviews, a few of which are quoted in this new trailer celebrating the Steam announcement: