On my site, I have a section where people can view a large image along with a few options underneath it. I placed each option in its own table cell and I reserved one table cell for the advertisement. The example is shown below right under the navigation bar.
As you can see, the advertisement is an image about "Hacking adwords". It's a advertisement of the size 336x280 pixels inside its own div inside a table cell (between
</td>). The margins around the ad is 12 pixels minimum like the rest of the ads are and the advertisement label on all ads are the same.
There is sufficient spacing between the ad unit and other text, and the unit couldn't possibly match up with other text in the table because the rest have a title in a box flushed to the top of the cell where as the advertisement has only the word "Advertisement" as a title in a completely different format.
This is the only ad unit in which I receive $0 RPM. I receive at least a couple cents RPM average on all other adsense ad units on my site.
Normally when I receive $0 RPM on a unit, it means something is terribly wrong with that unit.
Does Google not permit ads inside table cells? I haven't seen documentation on Google that states this.