Warren Buffett spent an unpleasant part of last weekend's Berkshire Hathaway (NYSE: BRK.A) stockholder party defending the company's investment in PetroChina Co Ltd ADR (NYSE: PTR), and its support of the repressive Sudan government. While he made the reasonable argument that in China, influence runs down from the ruling committee, not up to it, the pressure to divest holdings of companies that support the Darfur genocide is sure to continue.

But which companies are these? One often quoted source for suspect companies is the Sudan Divestment Task Force, a group formed in 2005 by college students concerned about the issue. They have compiled a comprehensive list of firms they believe support in some way the Sudan government. In partnership with Invested Interests, they have used this list to develop a screening tool that allows investors to check their stocks and mutual funds for such companies.

A complication to the issue might arise as Buffett makes good on his promise to donate most of his Berkshire Hathaway stock to the Gates Foundation. Given the Foundation's strong involvement in Africa and with the poor, as it begins to accumulate BH stock I'd expect it to ask hard questions about the PetroChina holdings. Ironically, it could be from this board that renewed pressure will be brought on the company to divest itself of PetroChina.

Addendum: after reviewing SDTF's Sudan Company Report which they shared with me, I can report that no U.S. based companies appear in it. The worst offenders include Petronas of Malaysia and several Middle-east based oil production firms.