Mine does the exact same thing. I posted about this a few weeks ago in the section specifically about the optical drive, and got a few responses back from folks that seem to know what they are talking about. They all seem to agree that it IS normal. I am still not totally convinced, but then again, I might just be paranoid. When I spend a big chunk of money on something, I tend to listen extra hard for any noises indicating that something might be wrong. So since I have apple care, I am willing to just wait and see. FYI, there are at least two different optical drives that I know of that come on the MBP. One of them, doesn't make the clicking noise, the other one (the one we have) does.
good luck