Yahoo's Domain Keys back at Google's Gmail?

Back in October 2004, Google started playing with Yahoo's Domain Keys anti-spam technology.  I'm not sure what ever became of that test.  What I do know is that sometime on Saturday March 4, (2006), Gmail started adding Domain Key data to the headers of Gmail.com-generated email that wasn't there before.  Here's a whole new text block that didn't appear in the header of Gmail.com-generated emails that arrived in my inbox prior to 3:30pm ET on 3/4:

=== BEGIN TEXT BLOCK ===== 

X-Gmail-Received: ea12dc925dc22bda63a450db9c41fd9106957597
Delivered-To: XMASKEDX@gmail.com
DomainKey-Status: good (test mode)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
        s=beta; d=gmail.com;
h=received:message-id:x-mailer:date:to:from:subject:in-reply-to:
references:mime-version:content-type;
b=ehQV0+Ql32iuvRwjBj8vE0EUJ4IlHfeDzPsKiWiYZZllqGkr77QgrSd
U1arT4mnfjruBweZn8Jva9o9faerdPzT0nhDFPwypXTIAbX30MmrAy
Rt4B858mD1QyKlsZTdEHZRwVWyajAu+IDHsgGkNIsb4z4ZAV50q4
crv+dYoFlA=

=== END TEXT BLOCK =====
Maybe this isn't news. Or is it?