The newer version of the M305 mouse can pair with the Unifying receiver, although it comes with a nano receiver. It has the Unifying logo on the underside of the mouse, but not on any of the packaging. Look for a 2011 copyright on the bottom of the package; the older 2009 version is not compatible with the Unifying receiver.
I purchased the K205 Keyboard which had a Unifying receiver. I already owned a M305 Mouse which has just a nano receiver. I attempted to pair it via the unifying software downloadable from Logitech and it does not appear to work. This is a shame since the keyboard and mouse both match. Thankfully I only need the kybd at the office so I will just plug the unifying receiver into the docking station I leave at the office and it will all be fine. I will continue to keep my mouse receiver plugged into my laptop usb port where it has been for months and use it when I am mobile as needed.