This has been brought up numerous times, so I thought I'd start a new thread so it can be popped into the WebmasterWorld AdSense Library for easy reference.
When the AdSense terms got changed [webmasterworld.com], it set off some pretty intensive discussion over what this multiple ad unit thing meant, if publishers could start using it right away, and how publisher's could control whether the multiple ad units shared any of the same ads.
That evening, AdSenseAdvisor posted the following: [webmasterworld.com] (msg #6)
The multiple ads language is confusing, I agree. At this point, there's no way to assure that ads won't be double-served if two layouts are on the same page. So multiple ad layouts on a page are still not allowed... I think the language is there so that we have room to introduce new features as we move forward.
Others have also received the same answer from AdSense Support, that only one ad unit is allowed per page.
The conference was a week after the new terms were released, so I specifically Gokul Rajaram (the AdSense rep who was on the Contextual Advertising panel with me) about this multiple ad unit thing.
Gokul said that allowing multiple ad units requires a change to the terms and policies. But because changing the terms and policies is pretty significant (and all publishers must agree to them to continue using their AdSense account), Google wants to update it no more than once per quarter.
As a result, the neccessary changes required for the use of multiple ad units was made to the Terms with the February 19, 2004 update, so that they wouldn't have to wait until the following quarter to make the changes to the terms. This would also mean they wouldn't have to make more than one Terms change in the quarter. It was not launched at the same time, however, because it is a feature that is still being worked on.
I then asked what the restrictions on using multiple ad units would be on a page. I didn't get a specific answer, but was instead told there are many publishers with content-heavy sites that would like to utilize multiple ad units on pages with lengthy articles, so an AdSense unit at both the top and the bottom of the article could be used. He did say they would not allow pages full of AdSense units, however ;)
So the big question is WHEN? Well, like the stats question, I didn't get a specific date, but I did get an answer of "soon". (The new stats arrived less than three weeks after Gokul told me the new stats would be released "very soon"). But, since Google only wants to update the Terms once a quarter, the timing could mean a few things for when we will actually see multiple ad units available.
On the latest possible date the changes will be made within three months of the February 19th date that the updated terms were released - meaning anytime between now and May 19th. Or it could be the quarter from January 1st to March 31st, meaning we could see multiple ad units within the next 11 days. Or they could be on Google quarter time, meaning their quarter could end anytime between now and May 19th ;)
The launch of the new ad channels might be a step towards multiple ad units, but there are also plenty of more variables for the script that AdSense could somehow use for this purpose as well.
I hope this helps clear up all the confusion, with some of the official AdSense voices on the subject. I will post if I hear any updated information from Google.