Using RFC1918 space also gets you an IP range where the outside world has no route to it -- Sorry, but no packets are not getting there, ergo no way to hack.
Assuming various things that should be standard procedure -- dynamic NAT as opposed to static, blocking source routing, etc.
Blocking source routing should not be standard procedure; as I stated earlier, source routing is much more valuable to me as a debugging tool than RFC1918 addressing is as a "security" tool.
At that point, just by use of simple routing, you've effectively eliminated 100% of attacks from the outside, and you only have to worry about inside. The front door is secure, now work on the back door.
100%, huh? You sure must feel safe, then. Good for you! It's a nice feeling when you have it. Stephen