This is a maintenance release. The following items were fixed and/or added:

Mac OS X hosts: fixed incompatibility with recent Mac OS X versions in 64-bit mode (bug #8474)
Mac OS X hosts: fixed incompatibility with hosts with more than 16 cores (bug #8389)
Mac OS X hosts: fixed painting corruptions on a second monitor in 64-bit mode (bug #7606)
GUI: restored functionality to set an empty host key to disallow any host key combination (4.0.6 regression; bug #8793)
VBoxManage: added controlvm screenshotpng subcommand for saving the screenshot of a running VM in PNG format
VBoxHeadless: fixed potential crash during shutdown (Windows hosts only)
NAT: built-in services use the correct Ethernet addresses in Ethernet header and in ARP requests
Host-only networking: fixed adapter reference counting
E1000: fixed rare guest crashes with Linux SMP guests (bug #8755)
SATA: fixed guest disk corruption under rare circumstances (only relevant for guests with more than 2GB RAM; bug #8826)
Storage: fixed data corruption after a snapshot was taken with asynchronous I/O enabled (bug #8498)
Floppy: several improvement
HPET: another fix for time jumps (bug #8707)
USB: removed assumption that string descriptors are null-terminated (Windows hosts only)
3D support: fixed a potential crash when resizing the guest window
3D support: fixed GNOME 3 rendering under Ubuntu 11.04 and Fedora 15
Snapshots: fixed another bug which could lose entries in the media registry when restoring a snapshot (bug #8363)
Shared Folders: don't stop mounting the other valid folders if one host folder is inaccessible (4.0.6 regression)
Linux Additions: check whether gcc and make are installed before building kernel modules (bug #8795)
Solaris Additions: added support for X.Org Server 1.10
Guest Additions: fixed inappropriate Guest Additions update notification when using vendor-specific version suffixes (bug #8844)