you can have up to five channels assigned to each single ad unit.
Watcht the adsense channel videos on youtube I linked to in the post before.
if you want to automate this it would require some programming.
Create 2 ad units the same size and color.
One gets the channel "ROS left col 160x600" and one gets both channels "ROS left col 160x600" and "blue widgets"
<if pages = blue widgets >
ad with channel ROS left col 160x600 + blue widgets here
ad with channel ROS left col 160x600
This way I show different ads with different channels in my different sub forums
It makes sence to really think about the naming system for your channels and what you try to accomplish.
There are so many different ways for using channels to get the best reports you need for your site(s).
You can use unique channels for every single ad or you can assigne multiple channels for multiple ads.
one channel 160x600 and assign it to all 160x600 ads
one channel left side and assign it to all left side ads
one channel above the fold ....
one channel ROS
this way your ad would get 4 channels and you could compare the performance based on location, ad size, targeting, above or below the fold and so on.
Another way would be to assign unique channels to each ad unit.
This is what I do.
I wrote an article about it unfortunatly I posted it on other websites first so I am not allowed to post it here.
"Google Adsense - The Secret to using channels":