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Hidden WebTV signatures

For those of you who don't read sci.crypt, Robert Ames <[email protected]> has 
posted an article in which he observes that all WebTV posts contain an 
X-WebTV-Signature: line containing base64-encoded data.  For samples of 
WebTV-generated posts, try the alt.weemba newsgroup, which is filled with 
WebTV-user drool (caution: remember to employ protection when exposting your 
mind to the content of the messages).  Some samples:
X-WebTV-Signature: 1
X-WebTV-Signature: 1
These decode into ASN.1-encoded DSA signatures, eg:
   1 30   45: SEQUENCE {
   3 02   21:   INTEGER
            :     00 98 20 B3 42 DF 93 AA DE 9F 35 7F 8B F7 E8 03
            :     08 40 9A A9 09
  26 02   20:   INTEGER
            :     0D 18 08 FC F9 B6 4F 39 99 48 2A EE 25 FF BB 8B
            :     D6 BC E8 06
            :   }
for the second one.  The key isn't included in the header, presumably the 
@webtv.net address can be tied to the hardware which contains some hardcoded 
DSA key.  I wonder if WebTV users know they're signing each message they send?