I'm considering running a virtual NAS. I.e. something like FreeNAS running in ESXi.
Google didn't turn up all that much on the subject.
Good idea? Bad idea? Suggestions? Hints? Setup?
Sorry to have to pull the familiar "it depends", but in this case it really does. What hardware do you have, what will the NAS be used for, what features matter to you, etc. Having a virtual NAS can be both a very good idea and a very bad idea.
That said, some thoughts:
The traditional weakness of virtualisation is disk IO when running from virtual disks. This will affect performance, and also there will be no real point in running RAID etc, unless you spread the virtual disk files over several physical drives.
If you can configure your virtualisation software to give the virtual machine direct access to the physical disks you will have good performance, but I would expect that you then won't be able to use those disks from any other VM or the host machine itself (I am not familiar with ESXi so you'd have to check the details).
If you don't care about stellar performance you can run from virtual disks anyway of course. There are some benefits to this as well, for example you would be able to re-create your NAS on any machine capable of running the virtualisation software, should your original NAS fail for some reason.