I've always been lousy at math, but yes, I've gotta go with zero being a number. My immediate thought was: If you can have negative numbers (and mathematicians use those), then surely zero must be included. Unfortunately, I don't have the dictionary definition of 'number' before me. But Scott above is obviously pretty knowledgeable in this area, and he says it's a number, so I defer to his judgment.
By the way, the ancient Maya also developed the concept of zero. They called it something else, of course, and it looked different from the Arabic (actually Indian) numeral, but the mathematical concept was the same, and with it, the Maya were able to make extraordinary (and extraordinarily precise) measurements and calculations.