In case you missed the official press release:

Developers who sell applications through Windows Marketplace for Mobile will receive 70 percent of the revenue from the sales of each application. In addition, they will be able to set the price for their applications in each market, maximizing their revenues based on targeted pricing strategies. Developers can also choose to distribute their applications at no cost. Up to five application submissions to Windows Marketplace for Mobile is included in the introductory annual registration fee of $99 (U.S.). Each additional submission within the annual period will cost $99 (U.S.). The registration fee is waived for student developers who want to reach Windows® phones customers, through enrollment in the Microsoft DreamSpark program.

So, for those of you who are students and are looking to get started in Windows Mobile development, sign up for DreamSpark and get crackin' :)