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).

Comments
I'm personally looking forward to OSX 10.10 "Sabertooth"
They may be running out of cat names, but I suspect the real reason is that Snow Leopard will be so similar in appearance to Leopard that they just used a similar name. All the news reports are saying that Snow Leopard is less about adding new features and more about making it simpler and more efficient while preserving the features they already have.