As Joe pointed out in his blog about the integration of XenApp&XenDesktop and CloudPlatform, one of the really exiting new features of the new XenApp & XenDesktop 7.5 release is its native integrated provisioning and interaction with AWS EC2. Allowing one to make use of the many advantages that such a public cloud can offer. Be it extending your existing on-premises infrastructures or ramping up quickly with little to no upfront costs or idle capacity.
Now from within Citrix Studio you can provision directly to EC2 you virtual desktops or applications. Simply create a new host connection using the Amazon EC2 option. Provide your AWS credentials (preferred would be to use an IAM user, allowing you to control and limit the access to AWS resources and services)