If it was only 51% (and why not just say >50% as there is nothing magical about 51) then it would be very problematic as has already been pointed out that could easily be reverted (as you have to continue to control the hashing power).

This is why any hard-fork should require a lot more % of the hashing power (it costs miners a lot in electricity to create new blocks so they are not going to want to be creating invalid ones).

If a >50% attack like this happened then there would be two Bitcoins and you could literally spend your coins twice. IMO if that actually happened then Bitcoin would lose 99% of its value (which is most likely exactly what the banksters are hoping to achieve).