Twitter suffered from some noticeable site availability issues last night, and is still running at a higher than usual latency this morning:
Twitter identified the cause of the incident as an "error with networking equipment", which caused the site to run noticeably slowly, sometimes even causing the fail whale to come out for a swim.
A few hours later, an update revealed that the networking problem had not quite gone away, although the site does appear to be fully functional now. Features such as searching, sidebar data and profile image uploads were temporarily disabled last night to try and stabilize the site's performance.
Netcraft's live uptime graphs for twitter.com can be viewed here.