A pulled WebmasterWorld thread has reports from some advertisers that Google is sending out notifications to them that they have changed their display URL policy again. This change is that you can have only one display URL domain in an ad group. So you can not have a display URL for www.seroundtable.com and www.rustybrick.com in the same ad group, according to this policy.
I have not seen confirmed reports of this email, but they are floating around the forums and blogs. Here is a copy of the email text:
In an effort to provide more relevant results and a higher quality experience for our users, we have made the decision to no longer allow multiple domains within a single ad group. Going forward, all display URLs within an ad group must be for the same top-level domain. Please note that this amendment to our policy applies to all advertisers, regardless of previous exceptions or acceptability of any campaigns. While we understand there are legitimate use-cases for multiple domains within one ad group, we ask that separate ad groups are created for any given domain.
For instructions on editing your display URL, please see https://adwords.google.com/support/bin/answer.py?answer=6272&hl=en_US
Please know that we will not be retroactively disapproving ads until the end of February. Please aim to make the changes before then.
The WebmasterWorld is on hold, likely due to a copy and paste of an email. But it might be activated soon and we can get more of an idea on the legitimacy of these reports.
Forum discussion at WebmasterWorld.
Update: We received confirmation that this will indeed be happening.
Update 2: On February 20, 2009, Google posted a blog post once again confirming this change and sharing that this will be going into affect the week of February 24, 2009.