I've been wondering the same thing. Then, recently I heard the following (I can't remember where; maybe it was even here): there are three milestones which mark toddlerhood--walking, talking, and cognitive development, as in showing a higher order of reasoning skills. My ds is 11 months and has been walking for a few weeks. I can also see his reasoning skills taking shape. For instance, he can figure out where I have gone if I leave the room. Before, he would just sit there and cry if I even moved 5 feet away. Now, he can follow me to where I've gone or where he thinks I've gone. My dh said that a few weeks ago I was gone, and he went all over the house looking for me. The first place he will always look is our computer room, because I spend so much time there. It's things like this that show that he is "thinking" a lot more and at higher levels.
But, he doesn't talk yet.