I just changed it to "Sent from my Mobile Device" That's just for my business email. That way they know I'm not at my desk, or not in front of my computer. I know this is getting more and more moot, but it's the idea that I'm not going to be typing back a Thesis...
My favorite is a friends: "Sent by carrier pigeon" Not very professional but makes me laugh every time she emails me from her phone.
I had "Sent from Outside Your Window" until the police confiscated my phone. I need to be more careful next time (well, in 5 years...or 3 with good behavior).
For my iPhone I use "Typed with one finger on a sheet of glass." My iPad has "Typed with two fingers on a sheet of glass."
"Sent from my AT&T iPhone". Just to get back at the Verizon Blackberry crowd (all 2 of them who might be left).
This one is a modification of "Sent from my iPhone" Sent from my iPhone [model name here] ([GB here], [carrier here], [iOS version here])
If it's not funny, then there's no point. The novelty has worn off and nobody cares if you're on an iPhone, android, iPad, iMac or MB Air. But mostly, any funny sigs should only be sent to friends.
Mine: Sent from my iPhone 4s. Blame all typos on Siri because I will A former colleague's: Sent from my Commodore 64