Holy cow. Is nothing on the Internet working these days? Facebook's acknowledged that a number of members have had account maintenance issues, and now Twitter has confirmed that "many" users are experiencing timeline delay problems.
Basically, the lowdown is that you can post tweets, and they'll publish, but that your timeline--the stream of updates from the Twitter accounts you follow--isn't bringing up any new tweets. For me, it looks like this started at around 8:00 a.m. PT.
Twitter, which has been prone to many an outage in its three-year history, says it is investigating the problem and will provide an update shortly.