So, snowballing resources?

Yeah, i'd imagine it'd be possible to land forces somewhere, you can't build sufficient AA to cover an entire planet =P

I think I could but I think there will be some sort of strategy that would counter it.

Giving that covering an entire planet with AA would be very costly, the attacker would probably have a gigantic army to break your defences anyway. As with most turtles, the defences are extended while the attacking army can all strike in a single point.
Not even counting the Rock beat Laser solution.

I don't know what I like the idea of less: the thought that a players' "back" planets would be safe havens, or the thought that an enemy can just dump units from space onto any planet, anywhere he wants.