I don't think so. Steve Jobs stated a couple years ago, when they announced that they would be transitioning to Intel Proccessors, that "Mac OS X has set Apple up for the next 20 years. -- Personally I think 20 years may be a bit long, but I don't think Apple will move on quite yet. Besides, can you imagine saying Mac OS XI?? That's just weird.
Oh, Jobs was exagerrating. Remember, Jobs did state that the name "Leopard" isn't actually final. Personally, with the introduction of so much new technology - the iPhone,
TV, 64-bit processing, Intel Processors, most likely new Apple Cinema Displays and of course those ever elusive "Top Secret Features", it seems pretty clear to me that OS X will be left behind for OS XI or something completely different.