It's live right now for players who own a non-Steam version of the game, and DICE say "The patch is going through Steam compatibility testing as soon as possible." Since it doesn't affect servers, pre-patch and post-patch players can play together. Some players are disabling automatic updates for the game in Steam, in the hopes of avoiding the patch when it comes. I'm not sure if that will still let you play, but right click the game and go to Properties > Updates if you want to try it.

Bad Company 2 patch fiasco