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.