Release notes: http://wiki.xensource.com/xenwiki/Xen4.0
VT-d ftw: http://wiki.xensource.com/xenwiki/VTdHowTo
New features for Xen 4.0.0 include performance improvements, better scalability, blktap2 for VHD image / snapshots / cloning support, improved IOMMU PCI pass-through for Intel VT-d and AMD IOMMU systems, VGA primary graphics card pass-through support to an HVM guest for high performance graphics using direct access to the graphics card, memory page sharing and page-to-disc for HVM guests, Netchannel2 for improved networking, online resize of guest disks without reboot/shutdown, para-virtual high-performance USB pass-through to both PV and HVM guests, and many other new features and changes.
From Phoronix
VGA Passthrough: 3D hardware acceleration inside guests.
