Good to know, I had no idea it was so complicated!
What about the baby einstein tapes? Like 'Baby first signs' or 'Words around the house' I use those with my son too, it is visual and auditory but also using sign laguage and Marlie Matlin (?sp) does the signing on these. Personally I find ASL a little addicting

I enjoying learning the signs myslef as much as I want my son to learn them, we even 'test' each other. I still get hung up on the alphabet sometimes and my son will manipulate my fingers to correct them for me