While Twitter is technically not down, the website has been experiencing very high error rates. In fact, I haven't been able to access twitter.com for about a half hour now without getting this message:

If you are experiencing the same issues, never fear. Twitter acknowledges the high error rates on the Status blog, stating:

You may experience some problems loading twitter.com and with Twitter clients. We are aware of the problem and are taking action.

I think the boring "Something is technically wrong" message is a lot more frustrating than the fail whale, possibly because there's nothing entertaining to look at.