Old article but good one...

Can baby sign language delay speech?

“Communication is communication. Signing, gesturing, using communication boards and other assistive methods are all acceptable in the very young child who is trying to get his message across and understand what others say to him,” says Wegner.

....

"Talking and signing together flood the baby with language," says Acredolo whose research, published in the Journal of Nonverbal Behavior, indicates signing may even give children a slight future verbal edge. "At 36 months, the [signing] babies in our study were speaking, on average, the equivalent of non-signing 47 month olds," she says.