Google announced they will begin lowercasing the domain and subdomains of the display ads for AdWords.

      Let me show you what this means. If I wanted my ad to have a display URL that looked like www.RustyBrick.com, I am out of luck. Google will automatically lowercase the display URL in my AdWords ad to www.rustybrick.com.

From --> To:
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      A Search Engine Watch Forums thread is not happy about this. One AdWords advertiser said, "I think Google is nuts, and disagree. I for one am upset and think we should voice our opinions over this bad decision." Another person said, "PenIsland.com is definitely better than penisland.com."

      Personally, I like lowercase domains and I had my ad always lowercase. But that is just a style thing, I am sure having uppercase letters in a URL would increase an ads click through rate.

      Keep in mind, the filename part of the URL can have uppercase letters.

Google said:

    As a result, we've decided to update the appearance of the display URLs of all ads that appear on search results pages in the next week or so. Following the change, the domain portion of your display URL will always be shown in lowercase letters. For example, if your display URL is Subdomain.Example.com/Subdirectory, it will appear as subdomain.example.com/Subdirectory.

      This will happen automatically and there is nothing you can do to stop it.

Source from Search Engine Roundtable