In this video from Citrix Synergy 2011, Gabe Knuth interviews Citrix sales engineer Andy Whiteside. Andy is showing off the new tech preview / beta of Citrix HDX 3D Pro which will be included in a future release of XenDesktop.
As we've written about before, HDX 3D Pro uses host-side GPUs for "real" apps that require GPUs, including DirectX and OpenGL apps. So this is not like RemoteFX which uses the GPU for 2D protocol encoding, rather, HDX 3D Pro is about things like Google Earth, CAD apps, medical imaging, etc. The main new feature in the beta is that while previous versions of HDX 3D Pro only worked for physical hosts (i.e. "Blade PCs"), the new version will work with XenServer's new ability to "pass through" a physical GPU to a guest VM. (Although that pass through is 1-to-1, like 1-GPU-per-session.) The new version will also support true multi-monitor.
By the way, that Nvidia multi-GPU box that Gabe and Andy were talking about is called the Tesla. (Although that's built to allow GPUs to be used like supercomputers. I'm not really sure if it will work for this use case.)