I've been using a USB (PS2 compatible) mouse with the PS2M adapter for months now. Ok - it's not true USB - but who cares?
I've got my mouse and wireless keyboard hooked up to the built-in USB connectors on my A600 DIY tower project.
If your tower/desktop doesn't have the connectors already built-in - Maplins sell a backplane with two USB connectors for around a fiver.