I will be the dissenting voice of the majority here, but from my experience I would not wait. My DD, now 3 1/2 had one word at 18 months. My pediatrician jumped on our case immediately. She said that children should have a minimun of 6 words (not including mom and dad) at 18 months. First thing we did, as an earlier poster said, was have her hearing checked. Please make sure you do that first. It was fine. Then we had her assessed by a speech pathologist. She concluded that her receptive language was ahead of schedule, but her spoken language was around a 12 month level. That qualified her for state sponsored speech therapy. She was also diagnosed with Sensory Integration Disorder (characterized by over sensitive to sounds, walked on tiptoes, very picky about foods, etc.) From 20 months to 3 years, my DD received weekly speech therapy and occupational therapy and it has made worlds of difference. She down receives speech therapy through our school district once a month, and that should end soon.
My point is why wait? If your child will "catch up" in his or her own time, then interventions won't matter, but what if s/he doesn't? Then your even further behind. Here in California, the state will pay for therapy until age three if your child has a large enough deficit. If I had waited until age two or three to see if her issues corrected itself, I would have received far less therapy than I did (our school district, which took over at age three is very limited with their offerings).
Every child is different but, please, have his or her hearing checked, look into what your state offers to young children with special needs, and see if you can have your child assessed by a speech pathologist as soon as possible.
He was crying when he got off the school bus today because his friend hit him on the bus. It think his friend was rough housing, but my son doesn't play that way. We teach people that it is not okay to hit, but deep down I was disappointed that he didn't just shrug it off. 
