It's mostly bug-fixes for the second VirtualBox 4.1 beta, namely regression fixes to the VMM and storage area. The source-code has also been fixed to support USB 2.0 support when creating non-official builds. Perhaps most interesting though is the experimental support for PCI pass-through, which is a feature for only Linux hosts right now.
PCI pass-through allows for improved performance of PCI devices on modern Intel VT-d and AMD IOMMU platforms. In fact, at least in theory, PCI pass-through in the virtualization world is designed to deliver near-native performance, particularly for networking. This IBM DeveloperWorks article describes PCI pass-through for virtualization in greater detail. KVM and Xen have already supported PCI pass-through on Linux.
See more details about VirtualBox 4.1 Beta 2 in the VirtualBox Forums.