Some things I'm sure you've considered, but on the odd chance you haven't:

Was anything new from Unstable installed recently? Is ariel always on or did you notice the problem on booting it this morning? Are there any trojans/worms/etc. that attack DNS that may be acting up?

As you know, things don't suddenly break for no reason. Something usually changes before the breakage is noticed.

When you've checked everything and not found the problem, it's usually something you haven't checked. :-)

Hope this tickles your graymatter a bit. Good luck!

Cheers,
Scott.