It went down about a week ago, was back for a short time a few days ago, and is back down. I've been trying to update two Wiis. I managed to do the first one when it was up, but now can't get two files for the second Wii:
HackMii Installer: Missing
BootMii SD Files: Missing
I believe these are
http://bootmii.org/download/hackmii_installer_v1.2.zip and
http://bootmii.org/download/bootmii_sd_files.zip. I've downloaded the first from a third party site, placed it in my temp folder, and ModMii extracts it and claims that it verifies. I can't find the second one though. (If/when the official site comes back up, I'll be deleting the download I don't entirely trust.)