See bug 329781 comment 16 and forward as well as bug 327300 comment 4. Messages from BlackBerry devices come across the wire as base64 encoded, and are stripped out as binary attachments due to Giganews' draconian policies. It's possible that other mobile devices use a similar encoding mechanism, or that other mail clients might. As per Dave's comments, mailman should be configured to reencode the messages as plaintext so that the content on mailing lists, Google Groups and NNTP is as in synch as possible.
I suppose if it makes you guys feel better since the summary is more clear (even though I've already said it's part of the same task) I'll leave this open instead of marking it as a dupe, and just set a dependency.
OK, this got fixed with bug 327300.