It seems Google has now disallowed pay per click domain testing within a single adwords adgroup. Apparently they feel it is confusing the ppc search customer … see this article http://www.seroundtable.com/archives/019276.html
Perry Marshall originally pointed out the Adwords Display URL was the second most powerful part of the entire ad (after the headline). Rotating domains and leaving all else equal in an ad test often produced gains of 100% or more in click thru (and sometimes also conversion).
Unfortunately, this now requires multiple ad groups.
However, there is a work-a-round which is also more powerful. I’ve been teaching this for approximately a year not because of Google’s rules (which have only recently changed), but because it’s faster, easier, and less expensive.
The short story is … test headlines until you come up with a great one, then make your headline into a domain.
For a full explanation with an illustrated example, please download the PDF Adwords Display URL Testing Hyperdrive