When I see emails with "Sent from my iPhone/Blackberry/Palm/etc.", I always respond in kind: "Sent from my seeing iMutt". This is just another signature to poke fun of. :)
If I wanted an ad in my signature, I'd put an ad in my signature. This is the sort of anti-feature that I'd expect from proprietary software, not from Ubuntu.
Unasked for taglines are why I ditched yahoo mail in favor of GMail.