The key highlights for VirtualBox 3.1 include teleportation/live migration support (the ability to easily move one VM from one system to another), VM states can now be restored from arbitrary snapshots, 2D video acceleration for Windows guests, network attachment type can be changed while a VM is running, experimental USB support for OpenSolaris hosts, significant performance improvements for PAE and AMD64 guests, experimental EFI support, and VirtIO network device support. The "significant performance improvements" for guests using an x86_64 or PAE (Physical Address Extension; 32-bit kernel but supporting greater than 4GB of RAM) kernel when using Intel VT-x or AMD-V with normal, non-nested paging. We have yet to run any virtualization benchmarks to see how these performance boosts measure up.
VirtualBox 3.1 Beta 1 also carries a number of bug-fixes and smaller additions, including support for some new OpenGL extensions (GL_EXT_framebuffer_object, GL_EXT_compiled_vertex_array) within supported guests and crash fixes for games. The change-log as well as download links for VirtualBox 3.1 Beta 1 is available from the VirtualBox Forums.