They could make a move for a Mac OS XI to show that they're beyond Vista. I know Mac already is, but people see OS X as having been out for almost a decade, around when XP came out, so they incorrectly assume it's old. I recently installed Panther on a small partition on my external and my cousin thought I was crazy for installing an OS that's two versions out of date ("That's smart, let me just install Windows 95 too >.>"). Imagine what people hear when they're told that Leopard (and 10.6 if it's made) are major revisions of an OS that's been around for ten years!
Also, not long after Vista came out MS announced they are working on the next operating system. People may look at it and at 10.6 and say "Windows is moving on. It's advancing. Mac is still on 10" when in fact 10.6 will undoubtably be more advanced than whatever MS comes out with.
Sure, "11" "Eleven" and "XI" don't sound or look as cool as "X" or "10," but they can't cling to it forever. Or maybe they can. What do I know.