I got my SpaceNavigator this weekend and was greatly concerned when it didn't appear to work. Then I remembered that drivers tend to need a reboot to register with the kernel properly. Fortunately that fixed things and I had an entertaining time playing around with Google SketchUp and other projects
Anyway, whilst Apple do say that a reboot is generally necessary, in this technote: "Installing an I/O Kit KEXT Without Rebooting"
, they also say that it is possible not to require one in some circumstances. You might want to consider enhancing your installer to include this functionality so that your users don't unnecessarily have to suffer the indignity of restarting their system.