I don't think it's an issue of being professional or unprofessional, but it helps convey to your coworkers and clients that you're on a mobile device. That can be helpful when someone wonders why you sent a short email or misspelled a word. At my "white shoe" law firm in New York, many partners and associates have iPads and it's very common for them to keep the "Sent from iPad" signature. The same goes for Blackberries and iPhones.
Yep, pretty much. I'm not sure how or why some think it is unprofessional. What image does it convey, other than the fact that you responded with your mobile device? Like you said, it's often important for the others to know that you are emailing from a mobile device. I've seen emails from law firm partners, CEOs and presidents on down send emails with the "sent from iPad" signature. The "Sent from my Blackberry Wireless Device" seems to have been around for years and years and is pretty well accepted in the business world.