I've read a couple of articles recommending adding clover to the lawn - apparently clover is more nitrogen tolerant, and hence more dog-proof than grass.

I need to reseed the lawn this spring, so I'm putting down a mix of a few different grasses and clover seed - we'll see if it stands up to the dogs, or not...

But the best long-term solution is to do as jrspet suggests, and train the dog to use a particular area.