While the site is down, the content might be accessible from the HTML dump posted by Scott Morrison and probably some of the content using the "usual methods" such as Google Cache or Wayback Machine.
EDIT: I have extracted files from the dump created by Scott Morrison and put them on my website. I do not make any guarantees for how long this copy will be available. (And if, for some reason, MathOverflow moderators prefer to remove this copy, I will do so.)
EDIT2: I should have mentioned that there is a redirect from tea. mathoverflow.net to mathoverflow.tqft.net - as mentioned by Scott Morrison in the comments to this answer. Of course, if there are some old links around, the redirect does not help at the moment - while the copy at the tqft.net domain is not available.