Usually a mouse-over will reveal what the logo is for, via a tool-tip. and Google has a history of changing its logo for special events or holidays or days in history.
However, i don't see any significance to why September 5, 2009 has any relevence to UFOs and no mouse-over tooltip like usual. all the search results are asking the same damn thing. is Sept 5 the history of Roswell? if so why isn't that made clear like all the other logo changes? the link if clicked on leads to a 'unexplained phenomenons' search which unfortunately doesn't clue me into why it's doing this logo change like this? perhaps, like the News, they wanted to get us all talking about it?
BTW the image if right-clicked and then 'save as' is clicked you get a "Go_gle.gif" filename, hiliarously ironic since the ship is abducting the second 'O'
Sheesh i think i'll stick to SeaQuest. at least no aliens there.