A Possible "Buy America" WTO Complaint

From Reuters:

Brazil may challenge the legality of a "Buy American" clause in the recently approved U.S. economic stimulus package at the World Trade Organization, Brazilian Foreign Minister Celso Amorim said on Monday.

"It's a complex legal analysis, but we're doing it," Amorim said.

...

Amorim said he thought Brazil could challenge the "Buy American" clause at the WTO, despite not being a signatory to an international agreement on government procurement.

I'm curious to hear their legal theory.