If you upgraded your system to Ubuntu 8.04, maybe you can't locate the correct rule to modify in the udev configuration file specified in last post (/etc/udev/rules.d/40-permissions.rules).
So open your /etc/rc.local script as root and add the following line to allow VirtualBox attach to your USB:
#USB Support for VirtualBox
VBOX=$(grep vboxusers /etc/group | sed 's/vboxusers:x:\(.*\):.*/\1/')
mount -t usbfs -o devgid=$VBOX,devmode=664,nodev,noexec,nosuid none /proc/bus/usb
Add these lines before the exit 0 command.
I hope this post may help someone,