This third beta of VirtualBox 4.1 doesn't offer up any new features, but it does have several key fixes. Built up over the past six days since the last beta are various GUI fixes, support for linked cloning, a fix in the PCI pass-through support for the ICH9 chipset virtualization, better configuration of storage controller attachments in VBoxManage, storage and networking fixes, WDDM Windows display driver fixes, and Mac OS X / Solaris host specific fixes. All the official changes are mentioned in this forum thread. There's also this mailing list announcement.
Generally with past VirtualBox releases while under the control of Oracle they have done about four beta releases before going gold, but regardless, VirtualBox 4.1 final will likely be out in the near future based upon their expedited development release cycle.