Too risky. It's a sad thing about our society but also think of the value. Stealing someones wallet you get maybe $20 to $40 in small bills and credit cards that are blocked within minutes of the mugging.
To make a livable income the mugger would have to risk stealing at least 15-20 wallets a week! That's a few muggings a day. Increases the odds something goes wrong, plus spending a lot of time finding victims at the right locations and times.
With an iPad, its a $500 to $700 device that can easily fetch it's full retail value or damn near it on craigslist or eBay. Steal on 64Gb 3G model and you got your money for the whole week! And now that everyone has one and uses them on trains and buses (especially people sitting by an exit) you basically have 80% of the job already done for you.
If I was a mugger, I'd be stupid not to target iPads or iPhones. That is why as a commuter, I never take out such devices unless I have to. If I'm listening to my iPhone I have it in my pocket with just the headphones out. Not easy to snatch, compard to someone sitting a few seats down playing angry birds on their iPad!
Remember, in the wild, you dont have to outrun the tiger to survive, you just have to outrun the guy next to you. Be smart.
Using your iDevice actively means you aren't paying attention to your surroundings (even if you like to think youd notice something while using it you really wont) and it makes you an easy and high value target.