Twitter's service has been down since about 9am PT for all users. As of about 10am PT, some users, including myself, have been able to access the site, although inconsistently. Twitter has not said when the service will be back or what the cause of the outage is.
"Users may be experiencing issues accessing Twitter. Our engineers are currently working to resolve the issue," the Twitter's status blog said. As of about 10:15am PT, Twitter is still experiencing a service outage for at least some users. Looking at Pingdom, this appears to be Twitter's worst crash in months.