I just read the following on Parents.com:

A recent study in Child Development showed that 2- and 3-year-olds argue with their parents 20 to 25 times an hour.... "Kids this age are realizing that they can assert themselves, and arguing with you is one way they gain confidence," says John Sargent, MD, a child psychiatrist and professor of psychiatry and behavioral sciences at Baylor College of Medicine, in Houston. Remember: The world is still a big, mysterious place to your toddler, and he feels pretty powerless in it. Saying no is a normal, healthy way for him to feel as if he has some control.

To be honest, while she is exhausting at times (especially when I'm trying to get the two of us out of the house), R hasn't worked herself up to her full arguing potential yet. I think right now we argue about 10, 15 times an hour. As the Japanese say: Lucky!