I've always found that people who assert that only whites can be racist are defining race differently than most people. The disctionary (and most people define racism as "prejudice based on race." Under that definition anyone, of any background can be racist.
A minority definition is "racism means prejudice based on race coupled with the power to make that prejudice a social norm." Under that definition it it very hard for a non-white to be racist, since he will rarely have the power to back his position up on a societal level. I can hate people who eat chocolate to no end, I have no power to make my hated of you "chocolate lovers" a socially accepted and widely practiced phenomenon.
I favor the former (dictionary) definition, but the latter one is not wrong, and the argument "Only whites can be racist" is a silly semantic one rather than a substantive one.
I do think it is true that, as a matter of western history, non-whites have suffered about 100x more because of white racism than whites have suffered as a result of non-white racism. If there were a term for some concept that meant "not only be prejudiced but that you be able to inflict suffering based on that prejudice," I think that term would be far more applicable to whites than to other ethnic groups (at least in the western world. So let's call that "de-facto-racism." If we accept the definition for the term, then I suspect it's fair to say that , in America 99.99% of the time, only whites are de-facto-racist. Of course we could just as easily call that term "socially dominant racism" or "racial powerism" or "super-racism" or anything else we want.