I would put my money on Cisco. They have been trying to reinvent themselves as a cloud company and clearly their financials show declining margins on hardware as the market shifts to cloud. Their intercloud announcement is interesting but not real yet and I think a Rackspace acquisition would give them a serious boost just as the SoftLayer acquisition by IBM has really accellerated IBM's move into cloud services.

The other possible buyer could be EMC -- for similar reasons -- but I would definitely think Cisco would be more likely.