Contained within the update? Fixes for all of the known bugs, 20 new levels, and a new music track. All of this will be free for owners.
On the PC and Mac side, the Steam version of Super Meat Boy will "contain 30+ achievements and possibly a little something extra for Steam fans." It is unclear if this means content from Valve products, the previously stated Mr. Minecraft addition, or something completely different.
What is known, however, is that "shortly after release [of the Steam version we] will put out a free level editor and level portal to view and rate levels."
The WiiWare version has been indefinitely delayed due to size constraints, but Team Meat is exploring a possible retail solution.