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.

Pricing is going to be a huge deciding factor.

Azure is expensive, but its getting better. They enabled full IIS in the latest 1.3 release. Now you can host multiple websites within a single WebRole and map subdomains to each site. Remote Desktop access and full admin rights removed many other restrictions as well.

SQL Azure still has the craziest pricing. $10/gb. Thats why we decided to go the 'NoSql' route and use Table Storage for everything possible.

We do and for now we'll be focusing on implementing rudimentary support for stuff everyone needs. To compete with Azure's Table/Blob storage I think a combination of S3 and SimpleDB (or an rdbms) would be the best option... Obviously we would like to make it as easy for people to get started with this as possible, so we'll probably end up wrapping it in some way convenient to our users - for now people should feel free to use these services directly from amazon.

The ability to instantly scale up/down is a feature at the core of our business and it will be available no later than our official launch.

Providing various caching options is also ranked highly on our feature list and will be available as an add-on (and paid) service.