There is a new report from Gartner describing the IaaS main key player who drive the IaaS adoption and evolution as well as comparing the strengths and weakness they have.
The 2012 version of the document can be found here:
Magic Quadrant for Cloud Infrastructure as a Service
18 October 2012 ID:G00237002
Analyst(s): Lydia Leong, Douglas Toombs, Bob Gill, Gregor Petri, Tiny Haynes
With a little bit of history from the old Gartner's reports we can see as well as how the market has changed: