VirtualBox versions 2.0 and above on Solaris hosts uses the new Host Interface networking model. To enable, simply open the Settings dialog of a virtual machine, go to the "Network" page and select "Host Interface" in the drop down list for the "Attached to" field. Finally, select desired host interface from the list at the bottom of the page, which contains the network interfaces of your system.
With VirtualBox 2.0.4 and above it is possible to use Crossbow Virtual Network Interfaces (VNICs) with Host Interface Networking but with the following caveats:
A VNIC cannot be shared between multiple guest network interfaces, i.e. each guest network interface must have it's own, exclusive VNIC.
The VNIC and the guest network interface that uses the VNIC must be assigned identical MAC addresses.