Sierra's Prototype, once planned to featured online co-op multiplayer, will simply be a single-player affair, according to Radical's Tim Bennison. He tells Team Xbox that the Prototype team decided it wouldn't "be able to pull it off with the same level of quality and polish that we're aiming for with the original single player story". You're going to be going it solo, sadly, but perhaps your friends will get some enjoyment out of pointing at the screen, commanding you to punch that guy or punch that other guy.
Bennison leaves the window open for multiplayer opportunities post-release as DLC, but hints that Prototype would have to do well at retail for them to pursue it. That should comfort those who are interested in paying, say, $US 70 or so for your games.
Inside The Mind of Prototype's Alex Mercer [Team Xbox]