All USB chargers use the same voltage (5 volts). If it is a USB charger, it's safe to try. A charger is a USB charger if it has a USB connector (either mini USB connector or a full size USB connector)
Amperage (current) is a different story (100mA vs 500mA), but they all err on the 100ma side unless automatically negotiated to output 500mA. Some USB chargers are intelligent (auto-negotiation circuitry) and some USB chargers are 'dumb' (just a 5V adaptor that happens to have a USB connector. Even for the unregulated USB chargers, due to Ohm's law, the fixed voltage limits the current to the charging capacity of the device.
The 'dumb' USB chargers will simply charge a BlackBerry 7280 very slowly, since the 7280 seems to prefer to auto-negotiate before it wants to accept the full current (500ma instead of 100ma). At least, that has been my experience with using an off-brand charger with my 7280.
The bottom line is that it's safe to interchange USB chargers with different devices -- but be prepared for slow charging on certain devices that like to err on the safe side. (This does not always happen)