So why in the world would anyone want to do this? Because it would allow a Citrix VDI (virtual desktop infrastructure) product to connect an ICA client device to a virtual XP/Vista desktop with a single pure ICA connection.
Let's take a step back. Remember that VDI solutions allow end users to remotely connect to back-end workstation blades or VMs in a thin client computing kind of way, allowing users to have the benefits of their own "real" workstation with the portability and security of a server-based computing environment. ( More in VDI here.) Remember too that Citrix is in the perfect position to add VDI capabilities into their application delivery software offerings, but they kind of missed the boat on that and instead hastily released a free product called the " Citrix Desktop Broker." (This is just an app that you publish on a Presentation Server which connects users to back-end Windows desktops, so the end user connects from their client to the Presentation Server via ICA, and then makes a second hop from the Presentation Server to the back-end Windows desktop.)
The good news is that Citrix's Desktop Broker application is just a temporary stop-gap solution. They are working on a "real" VDI solution which is currently codenamed "Project Trinity." Trinity will integrate with Presentation Server's IMA infrastructure to provide users with access to virtual desktops via the familiar interfaces. It will also allow for direct ICA connections from client devices to the back-end Windows desktops, hence the need for an ICA interface to Windows XP / Vista.
I'll have a more detailed preview of Project Trinity next week.