I went round and round with the same thing on mine, and while it was in for other repairs I had the shop replace the gasket that goes between the alternator and the block. It took care of 95% of the leak, but not all of it. In the end, I had some oil getting through the thread and it was taken care of with the addition of a few crush washers.
My point? Might just be an oil leak. But, if there are lots and lots of miles on the alternator it wouldn't hurt to replace the bearing on it while you have it off to replace the gasket, IMO.