There's a growing number of users, but they aren't centrally collected. I've actually thought about creating a "Who uses rump kernels" page, but haven't gotten around to it. No better time than now to do that:

http://wiki.rumpkernel.org/Info%3A-Who-Uses-Rump-Kernels

(don't expect it to be highly populated yet, I just created it and seeded it with the project I'm affiliated with)