The online features will be patched in when the PSN and PC versions are released.
Crystal Dynamics has revealed to GameSpot that the Xbox Live Arcade version of Lara Croft and the Guardian of Light will not be released with online co-op, voice chat, and online leaderboards when it is released later this month. The online features will be patched into the XBLA version on the release date of the PSN and PC versions.