For better or worse, if you want to develop for Windows Phone 7 and publish, you have to pay Microsoft $99 for the privilege. However, if you publish two games to the Marketplace, you can get that back in a rebate from big'ol MS. That rebate was set to expire...well soon (it was always a temporary offer). Well, both an extension and a firm date has been given: February 15th, 2011.
So if you haven't yet done so, either publish those titles and/or send in the rebate.
Source: Doris Chen's Blog