SQL Server RDS is built on Windows. Albeit a scripted to the 9's adaptation to make it act more Linux like (e.g., RDS can't connect to your AD, so they had to make some sacrifices).
The big difference with RDS is that you don't get the full gambit of clustering capabilities that SQL server supports. This is mainly due to the fact they don't have a shared storage model that SQL cluster failover requires.
EBS is certainly capable of implementing the shared storage that the database requires, and the quorum driver for wolfpack ought to implementable without a physical scsi device backing it.
At least until recently the MS SQL Server clustering was based on mirroring, which is capable but deprecated in future SQL Server releases in favor of AlwaysOn clustering.
The big difference with RDS is that you don't get the full gambit of clustering capabilities that SQL server supports. This is mainly due to the fact they don't have a shared storage model that SQL cluster failover requires.