The first two versions of Mac OSX were internally code-named Cheetah (10.0) and Puma (10.1). With version 10.2, Jaguar, the big cat names were part of the branding.
What followed was Panther, Tiger, Leopard, and now...Snow Leopard. Why the obscure "Snow" Leopard? A brief look at the animal kingdom will tell you they're running out of cats.
As Snow Leopard is version 10.6, we can assume the remaining three iterations will be Cougar, Lynx, and Lion, in some Jobsian order. Cougar, in particular, should be a fun one to brand; we're looking forward to that "I'm a Mac" guy being set upon by a thrill-seeking older woman (who might, ironically, be wearing something in a Leopard print).