yes, USB should be able to provide 5V - 500mA, at least that what's the spec's say.
According to the same specs the ports should also be current limited, so you shouln't be able to blow them up...
still, spec is spec and practice is practice... Best to be carefull when experimenting
You get 5A (with Max 500mA) from USB if you cut a manufactured USB cable and join to the RED (+) and BLACK (-). And it does work I had to do this to get power supply for USB 2.0 ports I installed via PCMCIA card, on my laptop as the card slot aint got enough cpacity for the 500mA that most USB devices require.