Amazon's EC2 offers Windows based VM's and Citrix has uploaded a pre-configured image of XenApp5 on 2003 server. So for a per cent charge rate for multiple metrics (Storage, Network, Capacity) you can have a highly available Citrix environment.
Amazon Elastic Compute Cloud (Amazon EC2) is a web service that provides re-sizable compute capacity in the cloud, it's basically a system where you can rent (Xen-based) virtual machines by the hour, and you only pay for what you actually use.
The template image from Citrix has XenApp 5 pre-installed, along with the two-user developer license key. It also has a desktop, a browser, and WordPad all pre-published so you can start playing with it immediately.
The community Citrix blog details more here.
A quick walk through video is here.
5 Quick steps and you have a XenApp lab - lets see where this one goes