My daughter started watching Signing Time (http://www.signingtime.com) when she was 10 months old. She loved the show so we watched it once per day, for about 25 minutes per day.
By the time she was 14 months old I could take her to the zoo and she could sign Monkey, Zebra, Horse and many other signs. Here is a video of her doing exactly that: http://www.flickr.com/photos/mikejenphotos/3467807751/in/set-72157604954739094
By the time she was 20 months old she had a huge vocabulary of hundreds of signs even before she could say all of the words.
Now that she is 3.5 we communicate via sign language all the time. That TV show has been influential in teaching her to spell and read by incorporating finger spelling.
Of course, I wasn't just sitting her down and leaving her there. I would watch with her (that's how I learned to sign) and incorporate real world items so that she could see that link between the signs on TV and her own things (like toys, food, etc) but that TV show was absolutely instrumental in her language skills. (It didn't hurt that she's a genius... I'm not biased...)
My second will be born in January and there is nothing that could convince me not to repeat the same process with her.