"AdSense for content publishers, who make up the vast majority of our AdSense publishers, earn a 68% revenue share worldwide.

We pay our AdSense for search partners a 51% revenue share, worldwide, for the search ads that appear through their implementations."

http://adsense.blogspot.com/2010/05/adsense-revenue-share.html

Does it match with the percentage we guessed/calculated years ago?

The AdSense blog post starts with the words "Today, in the spirit of greater transparency..."

Perhaps, in the spirit of greater transparency, Google will allow webmasters to publicly discuss their AdSense statistics (you're already allowed to reveal total earnings, but nothing more detailed).

Tom:

I looked at Google's earnings for Q1 and I estimated that the revenue share is 71%.

http://googlesystem.blogspot.com/2010/05/google-reveals-adsenses-revenue-share.html

http://www.webrankinfo.com/images/google/part-adsense-reversee-t1-2010.png
Share of Adsense revenues with editors, based on financial reports.

http://www.webrankinfo.com/actualites/201004-resultats-financiers-t1-2010.htm

It seems to me that the difference between 68% and the figures from the financial reports could be due to AdSense Premium Publishers.

Unlike AdSense for Content publishers (who we are told have always been on 68%), AdSense Premium Publishers negotiate their rate individually with Google. Naturally, it's not going to be less than 68%, and may well be a few percent more.

It used to be that you could apply to become a premium publisher if you had 20,000,000 page views per month (or 5 million search queries per month). Nowadays it's "don't call us, we'll call you". If Google sees that you're a major AdSense publisher, and they like your site, they might invite you to become a premium publisher.

Here's an interesting factoid that I hadn't been aware of. (Thanks, omnivorous, for bringing it to my attention.)

Google didn't disclose this percentage out of the goodness of their open hearts; they disclosed it because of the Italian competition commission, the AGCM.

The AGCM was investigating, at the request of the Italian Federation of Newspaper and Periodical Publishers, whether Google was acting in an anti-competitive manner. Under pressure from the AGCM, Google agreed to reveal the split.

More details:
http://searchengineland.com/google-finally-to-disclose-adsense-split-42221

If Google has never changed the revenue share then:

1. it should've been easy to (gu)estimate
2. I don't see why it wasn't disclosed earlier
3. it's likely that it won't change it in the future (at least not in Google's favor)