http://forums.steampowered.com/forums/forumdisplay.php?f=1182
The mods usually post down time announcements in the steam forums. I'd check there.
http://forums.steampowered.com/forums/showthread.php?t=784745
Both links useless--no sticky in the first link.
and the 2nd link--starts at 2009 and instead of current downtime.
No fault of yours, thanks for the effort and links, but Steam just can't seem to be bothered keeping it's userbase updated.