The first 2500 people who make an online purchase for Games for Windows between July 25th and August 1 will receive an email from Microsoft containing a code for 400 MSP- free.
This, apparently, is in celebration of the joining of Xbox.com, Games for Windows Marketplace and Games for Windows. The code that the winners receive can be redeemed from either of the three aforementioned sites.
Microsoft promises the codes will emailed to the winners four to six weeks after the contest closes.