They're on pretty much every flight now: Handheld credit card readers to buy snackies and soda. None have exploded, yet, but the FAA just issued special advisories to airlines, since they're powered by explode-y lithium ion batteries.
Now, the FAA says the airlines need approval from the FAA's hazardous materials division, and has asked them not to store spare lithium ion batteries for the readers on planes. A few airlines went through some special training to get the okay, but at least Delta and JetBlue don't carry spares or charge the readers on board at all.
The current rate for gadgets exploding on planes is about one every four months, says a former NTSB dude. Which isn't so bad, considering there are millions of flights happening in that time period. Still, I have the feeling this xkcd comic is going to stay funny for the reasons it's funny now for like another year, max. [NYT]