The claim came earlier this week from Microsoft engineer Terry Zink who said he discovered Android devices were being used to send spam as part of an international Android spam botnet.
Google got on the blower to ZDnet and denied it all. A Google spokesperson said that analysis suggests spammers are using infected computers and a fake mobile signature to try to bypass anti-spam mechanisms in the email platform they're using.
Of course a compromised PC connected to Yahoo Mail could insert the message-ID, overriding Yahoo's own Message-IDs and added the "Yahoo Mail for Android" tagline at the bottom of the message all in a deception to make it look like the spam was coming from Android devices.
It would be interesting to see Google's evidence, but so far it has not been forthcoming.