Tools will one day be created that will mitigate 51% attacks with real world solutions. In a worst case scenario we can always point nukes at the server sites identified as primary attackers.
Or better yet, point Aunt Tillie at them.
The way I understand it is like corporate ownership -- if you let more than 51% out of your control, someone might buy it up and take over. Since the BTC model hands out "assets" to whoever wants to buy more computers, it's possible for a takeover. Expensive, but possible.