From what I've seen, its okay by the cops standpoint as long as you're not the first one. They always seem to get the first one and give up after that. Unfortuatly, the first one usually is leaning over the wall and gets pushed over from behind. I don't know what they do about fining or if they let them go after its all done.
Now by the fans perspective, you should only rush the field in big games. This year, only Nebraska or Tech would be appropriate at home. I would however find it really funny to get enough people to rush the field at Kinnick when we win there. If their arrest and release record over there with the athletics team are any indication I'd doubt you'd get in any trouble (unless you have some herbacide).