Though.... The only free games on Steam are Source mods, And Game demos, So I'm not sure if its possible.
making a "free weekend" and selling the game for 1-2 bucks will guarantee you a Lot of players.... But you guys aren't allowed to sell the game Right? (UT3 mod/Copying C&C stuff).
Steam supports Indie games, the 2$ ones, don't think they'll like it free.