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If you are part of a discussion panel on possible future additions to AdSense, they will ask you to sign an NDA first. But I don't believe you do for beta testing.
This has been my experience as well. I was asked to do a panel once and I believe there was an online NDA you had to agree to.
As far as serving multiple ads, I got caught serving competitors ads. (I used to sit in the back of the school bus.) I was politely asked not to do this. I wrote back and told them about how my great sites now had extra inventory and could I serve multiple ads. They said ok. I do happen to send google a rather large amount of business. Something about lemons and lemonade comes to mind.
I'd guess it 1) blocks ads with certain keywords in the ad text and/or 2) blocks ads that target specific keywords (in ADDITION to the ones that are causing it to appear on your site).
There are several off-topic keywords I would **LOVE** to block - blocking the specific URLs does no good, since there seems to be an endless supply.
Imagine my outrage when I checked and saw all four ads on the page selling Ford cars, trucks, parts!
Totally OT ads. But wouldn't it be more productive if they just worked on getting the ads better focused on the on-page content?
I never would have thought to add "ford", "car" or "truck" to a list of keywords to be blocked.
Oh well, sort of happy with the blocking...but it seems that I am getting less of a CTR.
It's not an addition to the control panel though - I have to email them my "blocked" wordlist.
I'll let you know how it performs along with skunker (who's initial results seem pretty good apart from the drop in CTR, although that's not necessarily a drop in revenue if the targetting is better, based on the "smart pricing" model).
Skunker : did they offer you a different word list per channel?
I want to suggest keywords! Waaaahhh!
Remember when we all used to use the Keyword Meta Tag?
It'll never happen - too open to abuse.
In other words, on a page that included hints for Viagra, debt consolidation, and diet pills, Google would simply ignore the hints unless those keywords or keyphrases were in the body text. (As a further deterrent to keyword abuse, Google could serve PSAs when off-topic, high-bid keywords were found in the hints statement.)