As you probably already know, formulae for content delivery based on IP blocks are inherently clumsy. People can switch proxy servers to mask their origins. They might not even notice in the first place-- my French landlord uses a Swiss ISP that is part-owned by a subsidiary of a U.S. company.
Anyway, Googling around there appear to be a number of commercial databases which correlate ip blocks to countries. I also found http//www.idefense.com/Intell/CI022702.html (no endorsements whatsoever regarding its comprehensiveness or accuracy).