There's a great book (and also a video) on dog's body signals - it's called "Calming Signals" and it's by Turid Rugaas. Anyone who wants to know more about body language should look at getting that.
Like Red said, the yawning can mean different things. Dogs do yawn for the same reasons we do, but they also "stress yawn". Typically a stress yawn will involve some different facial muscles (tightness in the corners of the lips, etc.) but to most people they look pretty much the same.
Yawning is a way for a dog to send a signal to whatever is stressing them, trying to relieve that stress by calming the other dog/person. If your dog is worried about something and you start to yawn at them, they will gradually relax because you are giving them the calming signal. I've used this in training with one of my dogs that would get stressed on stays. She would start to stress pant or yawn, and I'd start yawning back at her. She would visibly relax. It helped us through a tough time.
There are a lot of neat body signals - beyond the obvious dominant/submissive ones, there are ones such as a dog approaching another dog by traveling in a half-circle instead of directly to the other dog, and keeping their eyes tipped away from the dog. These are signals that they're not dominant and not wanting to fight.
One of the problems with taking pups from a litter and then not socializing with a lot of different dogs while they're young is that they don't always learn these body signals well.
I really recommend the Turid Rugaas book or video for anyone really interested in understanding dogs.
Melanie and the gang in Alaska