Late last summer the author of the respected contextual advertising blog JenSense reported that as of an August, 2005 policy change AdSense publishers were allowed to place ads on error, login, registration, "thank you" or welcome pages. Here is a quote from the blog:

Updated: I checked with the AdSense powers that be, and you are now allowed to put AdSense on these kinds of pages.

However at the time of this entry, 6 months later, the Google AdSense Terms and Conditions still expressly prohibit this:

5. Prohibited Uses. You shall not, and shall not authorize or encourage any third party to: ... (v) display any Ad(s), Link(s), or Referral Button(s) on any error page, on any registration or "thank you" page (e.g., a page that thanks a user after he/she has registered with the applicable Web site), on any chat page, in any email, or on any Web page or any Web site that contains any pornographic, hate-related, violent, or illegal content;

So what is the real answer? I sent an e-mail to Google AdSense support and here is the pertinent part of the reply:

Google ads may currently be placed on error, login, registration, "thank you" or welcome pages. However, please note that Google AdSense crawlers aren't optimized to target ads to your site content when you place the AdSense ad code on types of pages that contain a limited amount of significant content or information.

So, yes, even though the Terms & Condtions expressly forbid doing so, it is permitted. Why the difference? Does it take lawyers that much longer than engineers to get things done? Or maybe it has a probationary status that could go away at some point? Or perhaps it means that for many practical purposes the policies override the Terms & Conditions? Google only knows.