From Slate:

When I’m in a tight spot, my BlackBerry always helps me out. It also sends a subtle signal to my correspondents that I’m getting a lot done. An e-mail that says “Sent from my BlackBerry” gives the impression that you’re on the move but still chained to work, e-mailing from the elevator. An e-mail that says “Sent from my iPhone” conjures an image of a doofus who wants you to know he has an iPhone.

Actually, “Sent from my Blackberry” always gave me the impression that I was dealing with a doofus who wanted me to know that he had a Blackberry. I removed “Sent from my iPhone” from my signature before I ever sent my first email from my iPhone for that very reason.