bi-lingual with help from sign languageMy daughter, now nearly 5 years old, learned to 'speak' with sign language before she learned to properly speak either of the two languages spoken in our household.
I (a native English speaker living in Germany) have spoken only English with her since the beginning (no matter what the others around me think!), and her father, a German, speaks German with her (though he slips and slides a bit, since we speak a good deal of English together, and he's not so consistent, which used to worry me but now doesn't). I had the idea of using sign language with her, and although my husband was sceptical at first, he eventually saw that it was working and got into it, too. Eventually, we found the experiment to be VERY worthwhile--that is, in our case, using sign language (based on ASL) turned out to function as a kind of 'hinge' between the two spoken languages--for example, I would say 'cat,' my husband would say 'Katze' and we would both make the exact same sign for 'cat', which seemed to help her realize very early on that there were parallel linguistic systems at work here.
She began using the first three signs ('eat,' 'drink,' and 'more') around 11 months, and we then began to teach her more signs little by little over time, according to what was appropriate/necessary/interesting to her. Now and then she even came up with a sign herself, if we'd missed something (she invented a sign for 'breastfeeding'!!). By the time she was around 17 months old, she had a vocabulary of 80 signs. After she began to speak (around 18-19 months) and put sentences together (around 22 months), the words were often either/or (English/German) and sometimes hard for others outside the family to understand, but she very quickly learned to pronounce words clearly, separate the two languages and speak only one language at a time.
She almost never mixes languages, but often has to ask for help to remember the word in one or the other language. She speaks both languages equally well--with the only big difference being that her German vocabulary includes much more of the types of things that Kindergarten kids say to each other, whereas her English sounds quite mature and proper and polite (!), since she hears it almost exclusively from me or from the books and tapes we have!