When I run:
alex@host:~$ sudo cat /dev/.udev/db/%2fclass%2finput%2finput6%2fevent6 | hexdump

It outputs the same info every time, regardless of whether the mouse moves. In fact, it outputs if the mouse doesn't even move.
This has to be some special configuration on your Ubuntu. Have you used any other howto's to configure your mouse? btnx will only work if you get mouse events from an event handler.