Update: InfiBase has ceased operations, but the analyses they are providing may continue. Stay tuned.
First, they have put out two posts on sites using Amazon EC2, with other cloud providers included in the last posting earlier this month. Here is their chart showing the top 500,000 sites by cloud providers. Note how close Amazon EC2 and Rackspace CloudServers (based on Slicehost) are in this ranking.
In another post InfiBase performed a deep dive into the processing dynamic of various EC2 instances, including which processors are being used and how they stack up. Here is just one of their great charts which shows that AMD processors are used at the low end of EC2 while Intel takes over at the very high end.
With the data they are previewing in their blog (see the full posts there), I am intrigued.