An American university professor has been teaching dogs, cats and horses to understand and use sign language for the past 10 years, according to this report.
She has had most success with dogs and teaches them one symbol at a time. At first she teaches them signs for commands associated with things they like such as food and toys, then trys to teach them more complex commands. She has managed to teach dogs to let their owners know when the phone’s ringing, and to indicate what their symptoms are when they’re ill.
One area in which sign language trained dogs would be especially useful, she thinks, is in search and rescue. She hopes to be able to teach the dogs to communicate such things as whether people are injured, their sex and how many of them there are. She is also trying to teach dogs signs meaning alive and dead.
There is also a simple form of sign language for deaf dogs, and there are suggestions for signs you could use with them for commands and to communicate other things on this page. This article describes how a deaf puppy learned signed commands, and this one gives more details of how to teach deaf dogs sign language.