I may have a solution it is a "bug" in the default X config created in Debian; the default config has two mouse stanzas, one for a a /dev/psaux and one for /dev/input/mouse0 It seems to me that: kernel 2.6.0 sends movement events to both devices, and then the mouse moves twice as fastnote that I use devfs (yes , I know is deprecated, but I like it very very much)
a.