Samsung Galaxy S phone USB device recognized by host VirtualBox OS but undefined in guest OS
Is the USB icon on the VirtualBox enabled on the guest OS. 

If that's disabled, the USB on the host side will not pass it through the guest, this requires the handset plugged in host side.

It's a small icon on the bottom of the VM window, near the hard disk where it shows blinking activity? Click on that, to enable USB pass-through.

Are you intending to flash the device from within a VM?