Tomorrow’s Washington Post (I guess they publish their articles on early on their website) features an article on Domainers and the money they make. While the article does not distinguish enough between actual type-in traffic (for generic domains) and typos, the author appears to have done his research: He picked up on a current trend to make more out of those parked pages and also covers other trends from the industry, such as traffic testing and the growing trend to invest in valuable domains.
The Web’s Million-Dollar Typos

Some interesting quotes:

Industry analysts estimate that roughly 15 percent of all Web traffic originates this way.

Google won’t disclose how much revenue it is earning from ads on these types of sites, but chief executive Eric Schmidt said in an interview last week, “It’s a lot of money.”