Matt from 37Signals says:
Let’s be honest. “Sent from my iPhone” really means “I’m not going to bother to proofread and correct this because it would take me an extra 30 seconds.”
But that's not what it means to me at all. Here's my translation:
Steve Jobs gave me a place where I could say anything, but set it up with a default that gives him free advertizing every time I send a message, and now I'm giving him free advertizing every time I send a message.
I don't see the lack of proofreading as a flaw; if anything, mobile e-mails are better, because they're shorter and they get to the point. (The ideal e-mail may in fact be a tweet.) But giving Steve Jobs free advertizing is kind of a dumb move. Nothing against the guy, I love almost everything he does, but I think literally every single person who's ever sent me a "Sent from my iPhone" e-mail has had either a tech startup or an acting career to promote. You can in fact use that free advertizing space for yourself; it's kind of the norm nowadays.
It reminds me of something Tony Robbins said at TED: "Decision is the ultimate power." The only reason your e-mail is Steve Jobs' free advertizing service is because Steve Jobs (or someone in his army of minions) decided to put it there, and you never decided anything different. Changing the default is near-effortless, but it's not something many people do; that tiny space between effortless and near-effortless functions like a vast interstellar voyage. It's very similar to the reason why I charge high prices for my videos; I've read the economics, and the gap between free and paying one cent is much bigger than the gap between paying one cent and paying one hundred dollars.