Google says no, that it was just a DNS glitch. Then why was there another search engine showing up?
This one demands an in-depth press release, I'd say.
Update: The consensus seems to be that someone else exploited a DNS mistake on Google's part. So not a hack, per se, but a 'taking advantage of.' Sister LifeHacker has a few more details.
Google unreachable for 15 minutes [LifeHacker]