UPDATE: Over at the Official Gmail Blog, Ben Treynor, VP Engineering and Site Reliability Czar, wrote that the problem was related to a server upgrade. They underestimated the load that would be placed upon the request server, and things got bottlenecked after that.
Today, Gmail went down around 3:45 EST and the world came to a grinding halt so that millions could take to their Twitter clients and Tweet about it.
You could access your Gmail account via IMAP or POP, if you already had it set up. Some were able to access their GMail via iGoogle".
While many are speculating the cause, Google is currently saying it doesn't know what the root reason is and that they're investigating it.
Google was largely immune to the recent cyberattacks on Twitter, Facebook and LiveJournal. It will be interesting to see what the cause of this outage was. Of course, Google has suffered outages before which had nothing to do with cybersecurity.