IBM's XML exam (141) is pretty much the most well-known of the XML exams that's on the market.
That being said, a search through Google turned up the XML Master exam from Infoteria Corporation, and an XML certification from Sybase. So there are others out there, but I don't know what portion of the market they command or how others look on them.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!