From an email I received:
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After fixing loads of bugs in this most recent release, one of our
engineers needed a change of pace and spent a lunch break doing
something festive and fun. He created a tiny "easter egg " in our
product, just for the holiday season and just for you. See if you can
find it!

We'll give you one hint: it's a function. If you think you know which
one were are talking about, submit your answer through this form:
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I've tried guessing, but ran out of guesses, I'm not terribly good
with spreadsheets but I'm writing some python code to generate
a spreadsheet with all possible =function() combinations...
whch I can then hopefully concat in one. I'm assuming the
function has no parameter something like PI() ... but I have not
decided how to concat all the combinations together.

Anyone got any ideas? Shoot me a tip here:
> http://spreadsheets.google.com/ccc?key=pfxvuOQaRPbUpLVeRwtFtFQ

Here's the 'official' announcement on the Google docs google group page: http://groups.google.com/group/GDSupdates/browse_thread/thread/b81afa8e010d992?hl=en

A pity its not a javascript version of =game() and =game("StarWars") (openoffice.org)

Did anyone find it already?

Has nobody found this yet?

Ah, after spending another 5 minutes on this I just found it!

Really? Wow!

Did you submit your answer?

I tried looking but I don't use Spreadsheets that much, so I gave up after a few minutes.

Anyway Good Luck in the Competition!

And hopefully soon you can reveal to us all what it is!

Yeah, I've submitted my answer. It's not very exciting to be honest!

I've searched for the answer (now that I know it) and can't seem to find anyone discussing it anywhere online, so for all I know I might even be the first to have found it!

I've asked about this in their new help forums:
http://www.google.com/support/forum/p/Google+Docs/thread?tid=637df07f159a281d&hl=en

It seems you're still the only guy who have found it, congratz.

Well I've still had no confirmation from Google yet. Maybe I should submit it again another 14 times... ;-)

I'll submit it for you! (Under my name :P)

Winner: Tony Ruscoe
Answer: GoogleHoliday()
Source: http://www.google.com/support/forum/p/Google+Docs/thread?fid=637df07f159a281d00045fd936b91cc1&hl=en

Congrats Tony!

Congrats Dude!

I pasted =GoogleHoliday() into a cell, and all it returns is "Happy Holidays".
Is that really all there is? lol. Or, do I have to change the cell format from the default?

Congrats anyway.

Congrats Tony :)

How did I miss that? :P
Congratulations!

Congratulations Tony! :)

> Is that really all there is? lol. Or, do I have to change the cell
> format from the default?

It displays "Happy Holidays" and links the cell to this festive "Collaborative Spreadsheet Art" video on YouTube:

http://www.youtube.com/watch?v=KpcgRlXe40k

Brilliant! Your discovery, I mean, not the Easter egg itself . . . But how did you find it in five minutes????

But how did you find it in five minutes????

Pure guesswork. I tried for about 30 minutes first with stupid guesses like MakeSnow(), BuildSnowman() and ChristmasTree(). Then a couple of days later, after I posted above saying "Has nobody found this yet?" I tried again. This time, I figured the function name would probably start with "Google" otherwise it could conflict with other standard functions. I just tried things like:

GoogleXmas()
GoogleChristmas()
GoogleWinter()
GoogleSnow()
GoogleSnowman()
GoogleHolidays()

and, finally

GoogleHoliday()

'More grunt, less elegance' :p

Nice work Tony, Congratulations