Mousing over, which usually IDs theme pages, doesn’t seem to be working. 
Doesn’t seem to be available in IE(7). Which makes me think it’s a showcase of HTML 5 (?). Looks like Flash, but it’s HTML.
What theme page? All I see is the regular Google page.
It’s a bunch of circles that bounce around then resolve to the Google logo – but when you mouse near the circles, they scatter.
Ah, apparently it’s their birthday. (Explanation, and the original heads-up on the page, should be credited to Stephen Fry, via Twitter.)
According to Searchengineland, it’s not related to the company anniversary.
Ah. Apparently it’s http://google.co.uk
You have to click on one bubble of each color to get the explanation. It’s totally worth the effort though – awesome.
I was originally sent to the UK one, but I checked the US one before I posted, and it seems to be there for me as well. (Firefox, if that makes a difference, which jjimm seems to think it does.)
How the heck are you supposed to click on them?
if you leave your cursor where the dot will be, you can click on it when it settles.
Not that anything happened when I’d done it, of course.
It takes time, but it can be done. I clicked every color (and each shade of every color, to be sure) and nothing happened.
If anybody needs me, I’ll be stalking Hal Briston for a while, after which I will have my revenge.
Thank you! Someone needs to do it and I’m all busy having an acid flashback
Huh. It is a mystery.
From here.
I hate you.
He’s finally taking revenge on us for…you know. 
I’d seriously considered putting something in the edit box that would give it away, but I decided to side with evil on this one. Totally worth it.
http://img62.imageshack.us/img62/386/cliph.jpg
Huh? What bubbles?
For those that aren’t seeing it, you need to have IE8, Firefox, Chrome or another browser that supports HTML 5. IE7 and earlier and other non-supported browsers will see only the regular Google logo.