I have super smart kids. Not like genius, but smart. My 4 yr old knew all the colors, shapes, letters (capitol and lower-case), sounds the letters make, and could count to 20 before his 2nd birthday. His older brother (now 7) was tested last October, at the beginning of 1st grade. The test they gave him started with first grade questions and went up to 12th grade. He answered as many as he could until they were too hard. In most subjects, the questions got too hard at 5th or 6th grade, but in one subject, he got up to ELEVENTH!!! As in junior in high school, and he was 6 at the time!
I try to teach him social skills, and I watch the hemline of his pants to avoid the typical nerd dress-code. But he still has a high "nerd potential." And you know what? Nerd isn't what it used to be. These days, with role models like Bill Gates, nerds are empowered and respected for their brains. Just like "geek" no longer means anything bad--it means someone who's good at computers.
I would suggest you search for Hoaggie's Gifted Page and go to About.com and read their articles about gifted children. I've had to fight my own mother's out-dated ideas about giftedness, the very ideas she used to raise me. For example, grade-skipping became a no-no years ago, but now it's understood better. My son skipped first grade and is doing GREAT in 2nd grade. He's not ostracized in his class. To the contrary, he's a little bit of a celebrity. The other kids take care of him because they know he's a whole year younger. And he had a bunch of them drive a long way to get to his birthday party.
I think it's odd that anyone would be frightened by smart kids, unless it's the sort of fear I have, which is that I won't be up to the task. My 3 very smart boys are demanding! We have in-depth conversations that tax my college education. My 4 year old hops into his car seat and asks, "Mom, can we talk about bacteria again?" He can explain the human immune response, the difference between bacteria and viruses, and how they infect our body cells. I worry about finding books advanced enough for my 7 year old that aren't too mature in subject and plot. And mostly, I worry that I won't be able to help them reach their extraordinary potential. I would never, NEVER dream of squashing their potential!
Reading about gifted kids may give you the information you need to talk to your MIL knowledgeably about the subject. For example, when she talks about social awkwardness, you can say, "You know, recent studies on smart kids say...." or "I just read an article about that very thing! Would you like me to email it to you?" that may change her mind, or maybe make her back off, either of which would be great!