The change has not been merged, and the bug still exists in the svn version. The distance the mouse travels on the screen given a physical motion should not change, regardless of fps. The current code in LinuxMouse::_processXEvents() iterates over the entire mouse event stack, however the stack only holds the most current relative motion event and not the sum of everything thats happened, so the lower the fps, the less actual motion is captured.
The fix is to move everything within if( event.type == MotionNotify ) {....} out of the while loop and use if ( XQueryPointer(display, window, &u1, ¤t_win, &u2, &u2, &x, &y, &mmask) ) to query the mouses state every frame instead, this is what SDL does in their code for example. The patch submitted to the bugtracker earlier in this thread shows what I did exactly.
Its a fairly serious bug... hopefully it will be merged at some point.