I have no affiliation with AppHarbor, but I do know a lot about Azure. I don't think it's really the best choice for running a platform like this, so I doubt they are using it. It's more of a greenfield platform because while you certainly can run legacy apps on it, in order to fully take advantage of the platform you have to design for it. For example. SQL Azure is a subset of what SQL Server 2008 R2 offers, so while if you use something like NHibernate, your app's data access code may still work, you may find that you have a reporting query that uses a specialized T-SQL feature it doesn't work.

Right now we run on top of Amazon EC2 Windows Instances.