Virtualization in the cloud is sort of an oxymoron because everything in the cloud is already virtualized, at least for the most part. If the cloud has an operating system, it is probably already running in the cloud. VDI in the cloud makes sense because users don't really care from where their virtual desktops or applications are being delivered as long as the usability is good.
Many of the Citrix products can be run as virtual machines on supported Hypervisors, such as XenServer, VMware, or Hyper-V. So, it makes sense to conceptualize that you can run many Citrix products in your private cloud or even in the public cloud, such as Amazon Web Services, Rackspace, or SoftLayer.
You can, and it has been done, build a complete XenDesktop desktop and application delivery solution using Citrix products in the public cloud. However, what you will find is that running a solution in the cloud can be costly in terms of money and performance.