In addition to those pesky details, a press release also reveals a new gameplay mode: "a special co-operative mission, designed to be played with a friend locally [...] This prequel mission allows players to play as Shank or his partner-in-crime Falcone, with deadly double-team moves and gameplay that encourages teamwork."
It sounds like the co-op is limited to a single, "prequel" mission, which is a bummer; however, Shank's charm has always been its good looks, and you can enjoy that all by yourself.
Update: By way of Shack, it looks like the official Shank site has added some details about the co-op campaign, which they say has levels and bosses really designed around cooperative play. The official site also dates the PC release as "fall 2010" so, that's something, right PC gamers? Check out some co-op screens in the gallery below.