Microkernels have been the wave of the future for longer than some of the most popular OSes have been around, namely Windows NT (not just Windows 7, but the whole NT design), Mac OS X (NeXTStep too, for that matter), and Linux (but, admittedly, not Unix).
If I may prognosticate, stripped-down monolithic OSes running on hypervisors are the wave of the future. Look at VM/CMS for an extreme example: CMS is about as complex as MS-DOS, being a single-user single-tasking OS with no memory protection or security model. VM is the hypervisor. Together, they date to about 1968, or a little before if you include research systems.
There are a lot of parallels between VM/CMS and a microkernel with a service built specifically to host an application. They are very different beasts at their core (a microkernel is built to connect services and a hypervisor to segregate them), but they solve similar problems in sometimes similar ways.
A Service VM is one where an application runs directly on the 'bare metal' provided by the VM (that is, the whole point of a VM is to multiplex the hardware; it provides few or no abstractions as such). There's no guest OS as you'd think of one.
In an exokernel, the guest OS is reduced to a library, like libc, which is (ideally) optimized for the specific application: Emacs has its own, Apache has its own, and so on. It's a half-step removed from the Service VM idea in that the applications themselves would still get to use the OS abstractions, unaware that the OS is basically gone.