I would hope, with FCP4 being less than 6 months old, that Apple would find a way to offer either a STEEP discount, or even a free upgrade, to owners of G5's. Of course, if it takes a year to get it out the door then that would not be the case.
The problem then, is whether Apple would delay shipping an update because of that. There are arguments on both sides.. if you bought FCP because you were happy with the way it worked on your old machine, then you don't necessarily deserve a free update to make it work better on a newer machine. On the other side, if Apple can, through a patch or plugin make it work better on a newer machine, I think that it is reasonable to expect them to do that. If it requires a whole new version to incorporate those improvements, then the regular upgrade discount should be good enough.