"Real time" can mean many different things.

And how "fast" and "real time" a desktop feels usually has little to do with the technically real time (or lack of thereof) of the kernel.

That said, Plan 9 has an interesting implementation of real time capabilities: http://doc.cat-v.org/plan_9/real_time/

Windows CE, apparently... ;)

But there seem to be some other interesting examples, like QNX: http://en.wikipedia.org/wiki/List_of_real-time_operating_sys...

Linux can be configured as RTOS too, but I doubt that it would improve desktop responsiveness.

RTAI and Xenomai can be considered real time Linux and both require application support. Applications don't just become real time, they must be programmed that way. Common Linux desktop application are not programmed this way.

RTLinux is more widely used than RTAI or Xenomai, but it's a completely different model, it's more like a hypervisor. Again, it doesn't help. While the OS provides the serivices, applications must be programmed specifically to use it.