The simple solution is to leverage XenApp Feature Pack with XenServer.

Then give our friends at Marathon a call and say "I want High Availability for XenApp".....and soon complete level 3 Fault Tolerance...all provided on XenServer virtualization.

For reference, I worked with a talented coleague of mine, Barry Flanagan to produce a series of videos back in late 2007 for posting in early 2008, with Presentation Server on physical servers protected by Marathon.

Barrys Blog and Gabes Videos

Fast forward a year + and check out this video of what you could achieve with the only solution for virtual techologies that provides lock-step protection of users and their apps.

Be sure to attend the Citrix and Marathon webinar March 18th!

Marathon and Citrix XenApp Webinar