Gundato: I had a lot of math and science classes, so laptop wasn't really a good idea for those.
But for my comp sci and engineering courses (that aren't PURE math :p) I tend to use my lappy.
And honestly, the professor was probably right. People DO tend to browse with their laptops. Maybe you only SAW one student do it, or your rationale is just that only one person did it the entire time.
At my university, a lot of professors eliminate the "need" for laptops by providing the lecture slides separately, this way the students can focus on the lecture and fill in any blanks.
Besides, think about it from the professor's point of view: You get the possible annoyance of hearing all these fingers typing and clicking, and you might have to deal with an idiot who asks a question because he didn't hear you the first time (because he was IMing or checking his Facebook for "one second"). Just a lot of headaches
That being said, I have only ever had laptops banned in "non-real" courses (damned Breadth Requirements), but usually they just aren't needed.
I sat in the back of the class, and she didn't bother to take notes at all. Ever. All she ever did was sit in the corner and play sudoku on her netbook, or browse websites. For 2 hours. Every time.
I should also point out that I am a History major, which requires an assload of research and note taking, and laptops are perfect for it. Taking 2 hours worth of notes by hand on medieval europe was hell until I got my laptop.