I think the point of a microkernel is that it strips away all but the barest elements needed to run a program. For example, if you're running a single program, you could possibly get rid of user permissions, the task scheduler and maybe everything to do with the file system.
I don't think anyone would really want to lose those features in a desktop environment.
Although I think it would be very interesting if I were wrong :)
The point of a microkernel is to keep the underlying structure small and uniform. What you put on top of the microkernel is what defines your operating environment--not whether or not it's a microkernel underneath.
For example, Gnu Hurd and Mach microkernel systems run a microkernel. OS X is a Mach-derived system, in fact, IIRC. NeXTSTEP was a microkernel, also Mach-based. Minix 3 is largely microkernel in nature.
Although I think it would be very interesting if I were wrong :)