On Sunday 1/13/2013 from approximately 7:00pm to 10:30pm MIT experienced a denial of service (DoS) attack. During this period external network connectivity to and from MIT was down for the large majority of the Internet. IS&T staff responded and service was restored by 10:30pm.
UPDATE, 11:20 p.m.: mit.edu is back up, although the co-gen page still shows the Anonymous message.
Revenge for Aaron Swartz's death? TechCrunch reports  MIT's Web site is down, although some testing here (ping and traceroute) suggests the entire mit.edu domain is no longer listed in DNS.
The Tech tweets :
MIT's network has been down since 7 p.m. EST. MIT staff are working on the problem.
TechCrunch also says doj.gov is down, which may be true, but the department's current domain - justice.gov - was up as of 8:15 p.m. See, for example, the US Attorney's office in Boston .
Back at MIT, Chia Evers tweets :
I'm actually at MIT, and using 3G for Twitter. Weirdly, we can get to Google.