According to Xbox Major Nelson's blog, From Dust currently holds the top spot for the most full versions purchased, ahead of Insanely Twisted Shadow Planet and Bastion. As the first title in the Summer of Arcade, Bastion held the top spot in the week of July 18th, but From Dust stole the position in its debut the following week and has held strong ever since.
This week, however, sees the arrival of Fruit Ninja Kinect, which may bring a strong following from its mobile fanbase. But I surmise that even it will not be able supplant Ubisoft's godlike title, despite how much I enjoy slicing fruit. Though people will need to purchase this game as well as the upcoming Toy Soldiers: Cold War, releasing August 17th for 1200 MSP, to receive Crimson Alliance for free, I don't think enough people own a Kinect to warrant the a full download of Fruit Ninja Kinect.
In fact, From Dust will likely beat out Toy Soldiers and Crimson Alliance as well. But is it the best title in the Summer of Arcade?