For those who are still dont know this, Google has ALLOWED MULTIPLE Publishers ad code in a page / website. As long as the maximum number of ad units per page will remain the same (3 ad units and 3 link units) Here is the link to the Official Weblog of Google Adsense http://adsense.blogspot.com/2008/07/sharing-your-ad-space.html
Yes this is NOT new, in fact if you will check the blog post date its July, 2008 but a lot of publishers are still not aware of this.