I've got the same problem, I think. Host = Windows 7 Pro x64, guest = XP Pro. I think it's because I have only USB 3.0 ports (externally, at least). When XP boots, the "New Hardware" wizard detects a USB controller but can't find drivers for it so it shows up in Device Manager under "Other Devices" and marked as not working. anything I plug in gets grabbed by the host, regardless of whether the VM window has focus.
My mouse is connected to a 3.0 port, but Player handles it differently and it works fine.
The whole point of this exercise it to continue to sync my Palm PDA with Palm Desktop which is no longer possible in the host OS.