prefer one connection over another

[Benjamin Kay <benjamin benkay net>]

I've configured two system connections, one wired and one wireless, to 
automatically connect. Both of them work great on their own. The only problem 
is when both connections are available (i.e. the ethernet cable is plugged in 
and the wireless network is in range) NetworkManager connects to both of them!

I can see how this would be useful in some circumstances, but in my case it's 
creating routing and DNS conflicts. How can I instruct NetworkManager to prefer 
one connection over the other? For example, how would I tell NetworkManager to 
try getting an IP address from the wired network first during startup, and then 
attempt the wireless connection only if that fails?