Windows® Marketplace Registration
To publish and distribute your apps on Windows Marketplace for Mobile, you’ll need to create a vendor account with Microsoft. Before you begin, take a moment to review the terms of the program.
Get a Windows Live™ ID.
Dont have a Windows Live™ ID? Signing up is easy! (If you have a Hotmail®, Messenger, Zune®, or Xbox LIVE® account, you already have a Windows Live ID.)
Sign the application provider agreement
What's in it? Read it first.
Pick your account type & fill in your details
We’ll need your business information, such as tax identification numbers to establish your vendor account. For companies, this information should come from an authorized officer of your company.
Pay annual subscription fee
Get great benefits for just $99 (plus taxes, if applicable). DreamSpark students register for free.
Not exactly 100% free. Just sayin'.
Edit: At least you can start developing for free... unlike with the iPhone.