Some people claim that the x86 processor family is more susceptible to buffer overflow exploits than the PPC family is. I don't understand all the details myself, but the basic idea is that in many cases the core issue of security vulnerabilities is that under certain situations, data that is supposed to be "information," but not "code," in that it is put in a place where executable code is not supposed to reside (i.e. inside a graphic file, or whatever), is allowed to act as code. The buffer overrun issue appears to be an area where a number of these exploit possibilities exist.
I read an article that said that Intel chips are more likely to attract hackers, because in order for the chips to work properly, Mac have to recode the OS therefore causing a loop hole which hackers can use - Only something I read but if its true, who knows?