While this is standard cover-your-assets boilerplate, the truth is you can charge your phone off of pretty much any USB charger out there, as long as the following things are true:
- The output specification is +5V DC, and at least 500mA. If it's more (700mA, 1000mA) you are generally fine; the phone will only draw what it needs. My HTC Magic's "official" charger is 5VDC 700mA.
- The data pins in the charger are shorted (most modern chargers will work. If an iPhone charges off the usb plug with its cable, it means the data pins are shorted).
If you use a car charger, do not get the cheap ones. Car power is a lot noisier than household current, and you need a charger that can survive crank (starting the engine).
If you live in a country where power is unreliable, you may want to charge off a UPS or other power conditioner.
I have charged successfully with a Garmin GPS watch charging brick, the charger from an older smartphone (FreeRunner), and the USB outlets at Southwest Airlines waiting terminals (although I'd turn off USB debugging and USB mass storage before using a public USB charging kiosk.)
If you don't trust it, don't use it! Phones are expensive.
If your device DOESN'T charge off of USB (Pandigital Novel)...check the plug size and the output specification.
If your device uses a funny plug (B&N Nook Color - it's MicroUSB, yes, except that their plug in their cable is longer), then you need that cable.