Clem has posted in the MythVillage forum, saying that The Mill's current absense will not be permanent:

Recently it was taken down for private reasons known to a few people and the who owns it.

It served the myth community since 1999 and has a current archive size of 4.44 Gig. What we feel is one of The Mill's points of differentiation is that the authors of each work had complete control over whether their work appeared or was removed from The Mill. We simply provided the resources for authors to make their work available.

It is not currently available to the community, but will be making it's reappearance ASAP. All content that it had at the time of going down will once again be available when it comes back up.

Currently there are only rumors about what those private reasons are. If there's more solid information available, we'll post it here.