I don't have as much background on allergies as a lot of the people who have posted here, but I wanted to mention one additional thing about peanut butter: the stuff is loaded with high-fructose corn syrup. I know we think of it as lunch food, not desert food, but it's so high in sugar (sucrose), it's really sugar butter with a little peanut thrown in for flavor.
And when kids have a whole lot of sugar really young, it effectively redirects their taste buds -- they start perceiving healthier sugars, like fresh fruit, as sour and develop lifelong cravings for super-sugary foods.
I'm not a purist about these things -- sugar has passed my own son's lips plenty of times -- but I am a little confused about the notion of deliberately introducing peanut butter. There are so many ways to get kids nut protein with fewer allergy risks and a lot less sugar. Why not wait?
My personal plan on peanut butter is to wait until there's a social need for it -- say, if all the cool kids in the first grade lunchroom are having PB&J sandwiches. There's really not a nutrition reason to start it sooner.
Hope that's helpful and not too lecture-y,
P.S. I also recall reading somewhere (forget where, sorry) that today's commercial peanut butter is much more processed than it used to be, and that this processing process (or whatever) makes the potential toxins much more concentrated -- hence the rise in allergies. If you're committed to introducing PB, you might start by looking in a health food store.