What about "10.5"? ... I just want to state that I find the cat names really not important.
I think it's important for Apple to have a catchy, more marketing-friendly name than just a number. Version numbers are boring, and just don't go over real well with consumers. Futhermore, Apple's trend of inching along with .1 updates, which used to be sensibly reserved for minor
(free!) updates makes the difference between the versions seem much smaller than it really is. It's really a marketing nightmare. Just hearing "10.2", "10.3", "10.4", most people would assume they're really the same thing, not MAJOR revisions like they are.
Windows' names have always been dumb, but they're still better than version numbers. And definitely better than barely-changing version numbers like OS X's.
We've got 10.5 Leopard, then quite possibly 10.6 Cougar and 10.7 Lion. After that, I really think they'll need to change themes. I liked Apple's old musical theme in the OS 8 days, although they were always just codenames. I think they could get away with using musical terms again. I also think musical terms could be very marketable. Although...it might seem too iPod-related, which could be bad.