Pharrell Williams... He's an hip-hop singer/producer, who worked also with Britney for her third album (I'm A Slave For You, Boys...) and many, many others. His first effort with Madonna was the Live Earth song Hey You, which isn't exactly what I call a masterpiece, and the leaked song The Beat Goes On is absolutely terrible.
Please mind once again that the possibility for Madonna to work with Mika is just a rumour, at least until their record companies release a statement! So please, don't be either too afraid or too excited! lol