I’ve seen some webmasters put ‘thank you’ note on the home page of their site saying

.. and thanks to all of you last month I received my $300 cheque from Google …

Reading it further I found out that he thanks them for promoting and linking to his site. There’s nothing wrong thanking your visitors, and it doesn’t violates any law (that I know). Nevertheless that thank you note violates Google AdSense™ Online Standard Terms and Conditions.

Confidentiality. You agree not to disclose Google Confidential Information without Google’s prior written consent. “Google Confidential Information” includes without limitation: (a) all Google software, technology, programming, specifications, materials, guidelines and documentation relating to the Program; (b) click-through rates or other statistics relating to Site performance in the Program provided to You by Google; and (c) any other information designated in writing by Google as “Confidential” or an equivalent designation. It does not include information that has become publicly known through no breach by You or Google, or information that has been (i) independently developed without access to Google Confidential Information, as evidenced in writing; (ii) rightfully received by You from a third party; or (iii) required to be disclosed by law or by a governmental authority.

You can not disclose how much (or how little) you earn from Google Adsense.