It's 2013. "X" dropped in, you know, March 2001. That's over 12 years (with no current end in sight, although who knows what will happen this year or next.)
Software vendors are known to skip version numbers, or deviate entirely from past history and ACCEPTED "wisdom" (Windows 3.1 -> Windows 95 -> 98 -> 98 SE -> Me; Windows NT 3.1 -> NT 3.5x -> NT 4.0 -> Windows 2000 -> XP -> Vista -> 7.) The folks in marketing have to flex muscle every now and then.
By the way...what's the average length of service for MacOS 8 & 9?
~ 2.5 years each.
Granted, "8" was really the follow up to 7.6, but Steve Jobs wanted to shutdown the Mac clone business, and did so handily by calling the next release OS 8.
Length of service for all System 7.x releases? ~ 6 years. So we're already double that with Mac OS X / OS X "10.x" releases.
LOGIC says they can name it whatever the heck they want to name it and the Apple faithful will buy it. Don't assume "XI" means anything. Perhaps "11" will come to pass, or maybe "XI" will be chosen simply to avoid confusing the masses, but it's all just marketing. OS XI 11.11. We're going all elevens (run for the hills!)
What do you suppose OS X means if they have to follow it up with "10.x"?
The X is more than "10". It also signifies UNIX, and the NeXT heritage that birthed Darwin (Mach microkernel based, BSD derived UNIX with Aqua Operating Environment). It shows that Apple has put extremely powerful workstations with a UNIX lineage on the desks of average ordinary computer users. It's an amazing feat of technological prowess, and a lot more than just a numeral that incremented beyond 9.
Compare OS X 10.8 with 10.1 and it's an amazing difference, and an entirely new platform (literally - they kept OS X naming after moving from PPC to Intel and are still there years after the transition was completed, marked by cutting off PPC legacy support in 10.7.)
It's all just a presumption until they make it happen. Don't get ahead of yourselves.
I think it's just as likely they converge marketing of iOS and OS X (not intended to mean a converged OS in its entirety) and call it something completely new. You know, LOGICALLY you'd expect APPLE to THINK DIFFERENT.
At some point "i" will be kind of passé if it isn't already. Perhaps by 2020 at the latest we'll have forgotten about these names.