We purchased the Baby Signing Time DVDs when our daughter was 10 months and by 11 months she was starting to sign. We started showing our son the Baby Signing Time DVDs from the time he was brought home from the hospital. He is now 6 1/2 months old and has started signing milk in the last week. It is amazing! He has understood the signs "change diaper" and "milk" for a couple months. He will be upset and I sign milk and he would get excited.
I believe it is never too early to start teaching your baby sign, I loved that my daughter could tell us what she needed, there was no frustration once she could sign, and I am excited for my son to be able to communicate with us.