It just occurred to me that the default signature on iphone emails, is probably the best advertising tactic i've seen in a long time.
Anyone who has used an iphone or received an email, sent from an iphone, will notice the default signature on any mail accounts is "Sent from my iphone".
At first it seems like a harmless little note, but if you consider how many emails are being sent from iphones all around the world right now, each one with that signature, then you can see what a brilliant idea it is. Everyone gets this subtle little ad for an iphone in their inbox.
'hey look how easy it is to stay in contact with my iphone! check me out emailing whilst sitting on the toilet'
It's not often you find people who choose to remove the default, because the iphone/apple brands are so strong that it becomes a badge of pride, as much as a signature.