I really never had the need of even thinking about USB support on VirtualBox until I bought an HP OfficeJet 6500 multifunction printer at home. I chose that printer because it has a network interface and it was (more or less) supported by HPLIP Linux driver: I thought it could work with some hacking with Solaris as well. Worked indeed in Solaris but there was no hope to have the scanner work with Sane (let alone the fax...). No problem, thought I: I'll run a Linux or a Windows on VirtualBox and use HP's software.
Well: HP Windows software does not install if it detects that the OS doesn't support USB. No way to go, even if at the beginning of the installation I chose "Network attached printer". No way. USB support is good news for everybody and as far as it concerns myself, I hope to be able to launch and perform HP's software installation.
Edited: I finally had the time to setup another Solaris host and, indeed, everything is working fine.