This was in recent news, based on Elance data..

After making an entry into the top 50 skills in Q1 2010, Google App Engine, the cloud-based application development platform, showed the single largest increase in demand with over a 10x growth quarter over quarter. With this increase, Google App Engine (#37) moved past Amazon Web Services (#40) as the cloud platform highest in demand in Q2.

From : http://www.websitemagazine.com/content/blogs/posts/archive/2...

Currently though, the rankings have changed a bit.. http://www.elance.com/p/online-employment-report-it.html