Mac OS X != Mac OS 10.
In the beginning, when OS X was announced, Steven Jobs was pronouncing it "Mac OS Ten". However, he (and everyone else) is now pronouncing it "Mac OS X".
Jaguar was, for all intents and purposes, "Mac OS 12", Panther was "Mac OS 13", and so on, even though they didn't change the under-the-hood version numbering.
Like other posters, I don't think they will ever go to OS 11, either. Personally, I think Leopard: 10.5 really equals Mac OS X 5.
At any rate, the version number is just marketing, and therefore unimportant; 10.2 -> 10.3 -> 10.4 was just as big an update as Mac OS 7 -> Mac OS 8 -> Mac OS 9. Development can just happen faster nowadays because OS X has a more modular core than the old Mac OS did.