The idea of seasonal or event-themed logos is not a new one but that's never stopped dotdotdotcomma nicking an idea before and it's not stopping us now, as you may have seen if you've been paying attention to the top left of your browser. In case you missed any of them, an exhaustive list is given below.
If you're interested, the next logo due to make its appearance is Winter Solstice 2010 on 21st December 2010 and probably a day or two either side.
Standard logo
Australia Day (observed this year on 26th January)
Ayrton Senna (died 1994) (observed this year on 1st May)
Epiphany (observed this year on 6th January)
Gilles Villeneuve (died 1982) (observed this year on 8th May)
GTA-IV released
Holiday
Pac-Man (observed this year on 22nd May)
St. George's Day (observed this year on 23rd April)
Winter Solstice (observed this year on 21st December)