Some people in the Arduino forum have thought of forming a group that would purchase a VID for Arduinites to use in personal projects. This is a pretty good idea and its probably the best way to avoid VID/PID conflicts. Between 65,536 projects, that comes to under a nickel per PID.
BTW I'm negotiating with the USB-IF to see if we can allocate a PID for non-commercial Arduino experimentation. the community could use this for DIY projects.let's see what they say