Trine 2 will support cross-platform multiplayer across PC and Mac, courtesy of Valve's Steam Play framework. The ongoing beta is also now available for Mac, meaning would-be cross-platformers can test out the new feature immediately ... that is, if you pre-purchased the title.
Note: While Trine 2 will also be available via just about every method of digital distribution known to man – including GamersGate, Direct2Drive, the Mac App Store, Xbox Live Arcade, and PlayStation Network – only the Steam PC and Mac releases will support cross-platform play. Sorry, Xbox players won't be buddying up with PS3 players. The release is still a vague "later in 2011" and if our Finnish Indie Dev Hunks calendar is accurate, there isn't much of 2011 left.