I just read this post on when to start teaching your child a second language, and as an English teacher and mother of a bilingual child I thought I'd put my two cents in... so here goes:
As a "linguist, (I hold a BA in English, and am working on an MA)" I would definitely agree with the "start early" advice - the critical age for learning languages is from birth to around the onset of puberty (10-12 years old). However, the author of the other article said something along the lines of "it's as simple as saying cat, then followed by gato," which is advice that not all second language acquisition scholars (let's call them "linguists" for brevity) tend to agree on.
Most linguists now agree that the best way to grow up bilingual is to have one parent always speak one language, and to have the other parent always speak the second language with the child. It makes for a much less confusing world for a child, and the child will be able to tell apart the languages much sooner. My husband and I are raising a bilingual child (Dutch/German), and at age two he barely ever uses any German with me, and is using more and more German with my husband. (We live in the Netherlands, so he hears more Dutch than German on a daily basis). But it is clear that he can already distinguish between the two: when I do use German, he looks at me funny and says "no, mommy! Don't talk like that!" 
Another tip for raising bilingual children: to make sure the child learns the appropriate grammatical patterns, the parents or others teaching the child must have either have very strong language skills or be a native speaker of that language. If your grammar isn't great, your child will be confused - you can teach them a lot of vocab, but he or she will never be a true native speaker of that language.
Also, exposure is key - if you don't have a lot of speakers of the child's second language users in your community, you can use CDs, kids' DVDs, etcetera to expose your child to the second language as much as possible.
All of this I found out because a) I'm an English major and it was part of my requirements and b) I married a German and we are now raising our son to be bilingual (German/Dutch). Since I am an English teacher, I am hoping our son will also learn English from a young age. Kids in the Netherlands have English lessons in school from the age of 6 nowadays, but I do know that most teachers who teach English in primary schools are not trained English teachers - so I'm glad I will be able to help him a bit more. I'm even thinking about volunteering to help brush up the teachers' English skills at his school when he is old enough to go.
Our son is only two now, but his Dutch is actually better than that of most of the kids in his daycare (which is not always the case with bilingual kids - some of them are slow starters as they tend to have trouble separating the languages). He is starting to use more of his German with my husband and my in-laws now. We live in the Netherlands, so he doesn't have as much occasion to use his German, but we have kids' DVDs in German, and we try to Skype his grandparents a couple of times a week. Also, we make sure to visit them, or have them visit us, at least four times a year. Our son understands every word of German spoken to him, but he uses it more when there are more German speakers around, of course.
Unfortunately, there isn't much quantatative research done on bilingualism. In fact, most of the "research" out there is not exactly scientific. But if you are interested, do go online and find books on the subject. Some of the books may be a little technical, so that's why I thought I'd give those who are interested in the subject the short(er) version...
Good luck!
Veel geluk!
Viel Glück!