The Team Fortress Blog has released info about the upcoming TF2 Heavy update:
In addition to the three unlockable weapons and thirty-five achievements for the Heavy, we’ve got a new game mode with five new arenas for it, a new Payload map focusing on more open spaces than Goldrush, and another popular community-made map [cp_steel].
A mini-site with details about the finer points of the update can be found here.