I love how Valve does all this crazy profit murdering shit almost on a weekly basis, and yet still makes more money than they are able to use on their projects. I guess that's what happens when you only have 220 people in the corporation total.
Actually, it's brilliant. A large portion of the cost of a game you buy at the store is done by the publisher. They absorb the shipping and distribution costs and tack on large amounts of profit. Valve has completely gone around that with DDL, distribution is dirt cheap when all you need is an internet connection. So they can sell these games at $2 (they probably work these things out way in advance with the developers) and still make a killing because it only costs them the overhead for servers and maintainance.
In addition, many of these games would barely get shelf space in a dedicated gaming store, much less in places like Wal-mart where you get half an aisle or so dedicated to games (and half the aisle is dedicated to WoW, the Sims, and PopCap games). They're either too old or too indie to be worth carrying. Valve has, by selling them at huge discounts, made these small games much more valuable by sheer numbers and will make a killing doing so, both for them and the developers.
As much as I hate to sound fanboyish, I love the marketing plan Valve is using. A lot of people are going to make a lot of money over the course of the event, and a lot of gamers are going to pick up old games they lost/sold or couldn't afford at full price. Everybody wins.