At least on Windows and Ubuntu (I don't have any Apple hardware to test with) - you need to first configure your virtual machine guest with an appropriate USB Filter to capture the device.
The easiest way is by opening the settings page for the VM you want to configure, and go to the USB settings. Click the device icon with a small green plus to have it pre-populate the fields for your ipod, though you will want to clear out all the fields except the Name, Vendor ID and Product ID. Disconnect your ipod and reboot.
Once you have finished rebooting, start up your VM and once it has booted feel free to connect your ipod - with any luck VirtualBox should intercept and capture the USB device and pass it on to your guest instead of allowing the host to communicate with it.
1What happens when you unmount it from OS X and then try to access through VirtualBox? – slhck – 2011-06-21T21:35:08.393
It is not listed in USB Devices menu. – Joaquim Oliveira – 2011-06-22T17:51:37.940