Something Different: Using your fingers correctly
Should you make your baby into a signing freak?
There's a reason why children who were forced to take spanish classes, math tutorials, and riding lessons when they were 3-years old don't grow up to be brilliant geniuses. There's a reason why playing Mozart to your belly button doesn't cause a baby Bill Gates to pop out of your womb.
Normal brain development is controlled by tightly-regulated-genetic signals and codes. Although there is room for plasticity, your ability to control your child's cognitive outcome is limited. If you think about it, there is a significant evolutionary reason for why this is so. If every environmental trigger had the capacity to drastically alter brain development, the likelihood of things going wrong would increase and endanger the survival of the species.
This is not to say that you can't bring a child who is working below their potential or who has brain abnormalities up to normal or above normal functioning. There are many ways to hijack systems or push systems to function normally when something has gone wrong. Many have seen your hard work pay off with your own children. Obviously, if your child has an impairment requiring them to sign as they mature, then starting early may be a good idea. If your child is "normal", however, I wonder about the cost versus benefit of such training. Most parents I know already non-verbally communicate with their infant, and children learn language by being exposed to it, by hearing it and being forced to use it. My concern is that focusing on non-verbal communication takes away from verbal skill development. Moreover, such training eats away at parents' funds; these lessons and books aren't free. Besides, aren't children supposed to be chewing on their fingers, releasing that viscous goo you find smeared on mommy's and daddy's work clothes? How can they sign and slime at the same time?
Loving your child and letting them be a child so they can develop on their own trumps over-zealous-parental-brain manipulation. I realize this isn't a very scientific analysis of the issue--I'm just feeling like keeping this on the non-hardcore side.