What do you mean by cumbersome to pirate? From all reports about their DRM servers going down, there's a mention that the pirates have theirs keep going.
You cannot stop a single player game being pirated, save for perhaps a very expensive and obscure hardware dongle, which can be emulated if the desire is there. What you can do is do CD-Key matching for online play, and use other ways of limiting gameplay for non-legit users.
I personally would just limit it to steam sales, and have retail packages in the same way that Valve do for their own games on PC that are sold in stores. It certainly doesn't stop anything completely, but it is acceptable DRM, unlike the rubbish in AC2 that promotes piracy.
The best DRM is something a legitimate user will never notice.