It won't be this decade. Maybe in 2011. But really, that's a stab in the dark. I doubt even Apple has a definite plan for it. Why write a brand new OS when the current one is working very well for them at the moment.
I only ask because the difference between XP/Vista seems similar to the difference between Tiger/Leopard. Yes Windows considered is a whole new OS. Mac seems to be treating it as an upgrade. I just wonder what makes the difference between a new version and a new OS in Apple's eyes.
When Apple released OS X they changed the way they number their OS's. OS X in all iterations (10.3, 10.4, etc) run off the same architecture. Therefore, until Apple changes the core architecture the numbering will most certainly remain 10.x