Wow, within just a few minutes after a big after shock hit Haiti, the Twitter home page was showing the Fail Whale, and it still does now (more than one hour later).
A lot of retweets happening at the same time seem to cause a system overload. Once again shows that Twitter is being used as a news source for a lot of people world wide.
Twitter is back, but doesn't mention the exact cause of the outage:
... A sudden failure coupled with problems in switching to a backup system ...