a late response here...
My son didn't say ANYTHING (not even mama or dada!) at 18 months old. We got him hooked up with our local Birth to 3 - they did the eval and set him up for speech therapy in our home. (1x week until he turned 3) Then he had a guaranteed place in preschool 2x week. This next school year he will go 4x a week.
He is still behind his peers in speech. I'd say we understand him easily (not having to think about everything he just babbled and make sense of it) about 60% of the time right now.
He is SO bright though. He knew all the upper case alphabet at 24 months old and by 25 months also knew lower case. (you could point to any letter and he'd identify it correctly 100% of the time at these ages). He had
maybe 20 words at that point but had the alphabet down pat. He knew advanced shapes like trapezoid and pentagon as well at about 29 months old. Granted, very few could understand he was saying trapezoid or pentagon... If it was a direct question "What shape is this?" people could figure out he was identifying it correctly, but if he just saw one and said the word, people wouldn't get what he was saying.
Because of his annoying obsession with my computer, we dug out an old computer and set it up for him to play/learn on. After having computer access for 3 days he knew how to turn it on, open the specific game he wanted (there are a few educational games on there for him to choose from) and even figured out "click and drag" all on his own! He
completed the Reader Rabbit Preschool game in one week and is now working on Reader Rabbit Kindergarten. (He's 3 1/2 now)
He is also learning to read already. He loves his Hooked on Phonics books!
I'm not saying all this to brag on him (well, maybe a tiny bit. I AM proud of him after all...
), but my point is that delayed speech does not necessarily have ANY bearing on cognitive ability.