Eh, there have been several of these "distributed supercomputing" projects, most notably setia@home (which was one of the first major ones recognized by the media), and I can't say that I've heard about anyone complaining about security. If it's anytihng like set@home, you have the option of letting it automatically go online to get a new chunk of data, or manually doing it. Really, I don't see there being any security issues if you just do it manually.