There has been lots of research on the effect of using augmentative or alternative communication methods (like signing) on the spoken language development of children. However, I've never seen any that suggest that using sign or other methods delays speech -- in fact, just the opposite. Learning to communicate in any form develops the language areas of the brain, so that when the motor systems are ready for verbal speech, words and meanings have already been established. All that's left to do is to associate sounds with them. Kids having a head start in using sign have an advantage when they do begin talking.
There has never been a case in all the literature of a child who preferred to sign (or use any other non-speech system) after developing normal speech. Remember - communication is inherent! It's in our DNA. We want to communicate. Using ASL with children of any age only assists and advances this process for all involved!
Photo by Axel Bührmann via Flickr