Here's a link to a recent discussion of that very same topic:
http://groups.google.com/group/microsoft.public.excel.misc/browse_thread/thread/e0d563106f3d908f/98aed4413d44708e?q=%3DSUMPRODUCT(--(A1%3AA10%3D%22x%22)%2C--(B1%3AB10%3D%22y%22)%2CC1%3AC10)+&rnum=1#98aed4413 d44708e