There are some significant changes in AdSense Terms like you can now have a search box on pages that have no other content, your AdSense earnings could go to charity if you are inactive for a long time, you may show ads on 404 pages and so on.
Here’s an exact summary of the recent changes in the AdSense Terms and Conditions document (or download this side-by-side guide):
1. You can display Google AdSense ads and Referral buttons on error pages and registration pages as the following line is now deleted from AdSense terms.
[Deleted - Prohibited Uses] (You shall not) 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*.
*this may be a hint that AdSense for Email program is in the pipeline.
2. You may place the AdSense for Search box on web pages that have no other content.
[Deleted - AdSense for Search] Each Web page(s) that contains a Google Search Box must also contain other content related to Your Site.
3. You may place Google ads associated with multiple AdSense accounts on the same web page.
[Deleted - AdSense for Content] You agree that while You may display more than one (1) Ad Unit on each Site Web page, You shall not display any Ad Unit on a page that contains Ads associated with another Google AdSense customer (e.g., Your Web hosting company), unless authorized to do so by Google.
4. AdSense Referral products (like Firefox, Google Pack, etc) can be placed on web pages that contain no other content.
[Deleted - Referrals]: Each Web page(s) that contains a Referral Button must also contain other content related to Your Site.
5. If you are participating in any of the beta AdSense features (like AdSense for RSS Feeds), you should not talk about them on your blog.
[Added - General Beta Features]: You shall not disclose to any third party any information from Beta Features, existence of non-public Beta Features or access to Beta Features.
6. You can now have ads from other contextual advertising networks alongside AdSense. Similarly, you may have other site search from other providers in addition to Google.
[Deleted - General]: If You have elected to receive content or Site-based Ads, You further agree not to display on any Serviced Page any non-Google content-targeted advertisement(s). If You have elected to receive Search Results on any Site(s), You agree that Google will be the exclusive provider of Internet search services on such Site(s).
7. If your AdSense account is inactive, Google may donate your accumulated earnings to charity.
[Added - Payment]: You acknowledge and agree that Google may, without further notice to You, contribute to a charitable organization selected by Google all funds, payments and other amounts related to the AdSense Program that are held by Google and that are due to you (if any), but which Google is unable to pay or deliver to You because Your account is Inactive.
Important: If you are an AdSense Publisher, you are required to accept these new AdSense Terms and Conditions before May 25, 2008 else you may not be able to continue using your Google AdSense account.
Disclaimer: The above is based on my understanding of the AdSense terms document. I may have misinterpreted some of the lines so please consult your AdSense rep before taking the next step.