As I told you on another post I had some problems with VirtualBox on a Solaris host: missing USB support not only meant that USB devices were unmanageable, it also meant that other software (such as HP's solution center for multifunction printers) wouldn't even install because of that missing feature. I planned to use my printer though the Ethernet interface but the software wouldn't install.
Until I installed Sun xVM VirtualBox v. 2.2. Experimental USB support was sufficient for HP's software to install and the printer worked perfectly both from Windows and Linux guests. Direct access to the printer by setting up a filter was straight forward.
A note about networking configuration: HP's solution center uses a certain number of TCP and UDP ports to communicate with the multifunction device. The printer documentation was detailed and setting up a firewall or a set of NAT rules wouldn't be hard. By the way, once I realized that the problems I was experiencing with the scanner were due to VirtualBox's default network configuration, I decided to change the guest's network settings.
If you're a Solaris Express user who regularly updates its system, you'd probably read about Crossbow project. Crossbow project, which was integrated in Nevada build 105, aims to provide the building blocks for network virtualization on Solaris hosts. The first thing I thought about was, indeed, creating a virtual NIC. But the solution was easier than that and it's called "Bridged networks" on VirtualBox's jargon. You simply change the adapter configuration for you VirtualBox host from NAT to Bridged and you optionally choose the physical NIC you want to bridge upon, in the case your system has more than one. Boot your guest OS and you'll have a virtual NIC at your disposal, without the limitations of the NAT configuration. And if you are communicating with the "outside world", such as a network multifunction printer or some CIFS client, your guest OS' NIC will appear just as a physical NIC.
The only caveat to use this technique is that, on Solaris hosts, Virtual NICs and VirtualBox bridged networking isn't implemented (yet) on top of a Wireless NIC.