It's actually very serious. Just like ATMs, power stations can look completely harmless while being incredibly dangerous under the surface. Once you connect to a system via USB, it automatically allows data transfer, which makes your smartphone vulnerable to malware and data theft.
I can't speak for Android, but iOS 7 now has an automatic warning when you connect to new devices that asks if it should be trusted, presumably to mitigate this risk. However, I doubt anything less than a full hardware solution can actually eliminate the risk entirely.
There's a reason why conferences like DefCon warn you explicitly not to use power stations or ATMs nearby - they can be hijacked very easily.
Well, you should assume that everything out there would be malicious, and work from _that_; if you go to DefCon specifically, this is a reasonable expectation.
New-ish Androids pop up a "USB connected, do you want to use this" prompt as well; but many devices (esp. embedded ones) don't.
I can't speak for Android, but iOS 7 now has an automatic warning when you connect to new devices that asks if it should be trusted, presumably to mitigate this risk. However, I doubt anything less than a full hardware solution can actually eliminate the risk entirely.
There's a reason why conferences like DefCon warn you explicitly not to use power stations or ATMs nearby - they can be hijacked very easily.