To enter, vote for World Of Goo on the wishlist (bonus points for leaving a positive comment there) and post here that you've done so.
Theme Hospital will be given away, not on a particular date, but when World Of Goo reaches 600 votes on the wishlist.
As of the creation of this topic, the votes stand at 447.
Be sure to vote for these other quality games:
Aquaria - Bastion - Braid - Cave Story - Limbo - Super Meat Boy - Unepic - Vessel - VVVVVV