Steam and Runic Games have just added Torchlight 2 to the Steam Store. It is $20 for a one copy or $60 for 4 copies, so you get a 25% discount for buying in bulk. The multiplayer mode supports 4 players, so the 4-pack is perfect for anyone that has a lot of friends they want to play with. One copy gets added to your account with the other 3 copies added to your account as gifts. You can give these to your friends or even use them in trades for other Steam games.
On Steam every prepurchase has a bonus for paying before the game is out. In this case, the bonus is a free copy of Torchlight 1. If you already have Torchlight 1 you can gift the free copy to a friend or trade it with other gamers for a different Steam game. This prepurchase bonus also stacks with the 4-pack bundle; you get 4 free copies of Torchlight 1.
Torchlight 2 is also available from Perfect World Entertainment, the majority owners of Runic Games. However, Runic Games currently is not selling the game directly. If you want to support the developer more, you might want to wait until they are accepting orders directly. No matter where you buy the game, you will have to create a Runic Games account to play online.
The Steam page lists the game as a Summer 2012 release but gives no concrete date. I will wait until then to prepurchase. I’ve been burned in the past with games I preordered and the release window got pushed back further and further. I eventually decided I will not do preorders unless the game has a concrete release date announced by the publisher and developer (retailers just guess on their store pages). Hopefully, they announce a release date soon.