Here's info. on disc brakes: http://en.wikipedia.org/wiki/Bicycle_brake
From what I read and measuring my own rotor, the ISO standard for disc brake rotors is 44mm BCD (Bolt Center Diameter). Some Chinese brake rotors have a different BCD. You also have to be careful with the thickness of the Chinese disc rotors. Some are very thick, especially the ones made for scooters.
Here's a disc brake converter from Chopperus.com that has the bolt pattern for both ISO standard and non ISO standard disc rotors. I believe Chopperus.com house branded disc rotors are non ISO standard.
Stick with Avid, Hayes, Tektro and other name brands and you'll be o.k. Many of the sellers on Ebay from China will state compatible with Avid, Shimano, Hayes.
Just be aware of what you are buying.