There are many rumors about today's Google Balls logo, but we know it is not birthday related. Google in the past have celebrated their birthday on September 7th but not anymore. Danny has good history on Google's Birthday over here.
This year, you can expect the 12th birthday logo to be live on the 27th at google.com/logos/12th_birthday.gif. In fact, when it goes live, you will see the logo below:
So you have 20 more days to wait for it. Until then, enjoy the Google Balls logo.
Forum discussion at Google Blogoscoped Forums.