For the past day, Google's main logo has been replaced with one featuring a flying saucer.
Clicking on the image pulls up a search for "unexplained phenomenon," but there's no discernable reason why Google is marking today and yesterday. The UK's telegraph did a short piece on the trend, but I think their speculation about marking the UK release of District 9 is off- not only is Google more apt to mark an American release, but I've never known them to mark the release of any movie with a logo change.
One likely reason is in commemoration of the 44th anniversary of UFO sightings in Exeter, NH (full story after the jump):
An extremely well-documented and investigated UFO case of close encounters occurred in Exeter, New Hampshire in 1965. This case would make a huge impact on Ufology, and mainstream media as well. The Exeter events would be chronicled in an excellent book by investigative writer John G. Fuller, titled, "Incident at Exeter." Look magazine also ran a two-part series on the events, bringing the case to the minds of millions of people.
A Light Becomes an Object:The events at Exeter began as eighteen-year-old Norman Muscarello was hitching for a ride on the cold night of September 3, 1965. He was on Route 150 heading for the small New England town of Exeter, population 7,000, at 2:00 AM. As he walked along, he suddenly noticed an unusual light glowing in the dark skies. The light soon became an object, and headed straight toward the hitchhiker.
However, the logo change will have to remain as unexplained phenomena for the time being. In the interim, you can read about the Top Ten Unexplained Phenomena here.
Edit: An Associated Content blogger points out an encrypted Google tweet. The plot thickens!
Even more curious is the unexplained phenomenon of a Twitter post by Google. An encrypted message that read "1.12.12 18.104.22.168 15 1.18.5 22.214.171.124.14.7 20.15 21.19" was sent out to followers of Google on Twitter. Decoded, the unexplained phenomenon message reads "all your o are belong to us". Although the message is far from grammatically correct, it fits with the UFO that is beaming up one of the Google "O"s in today's Google images.