An interesting article about bilingualism in The Observer today, showing the advantages of being bilingual.
http://www.guardian.co.uk/technology/2011/jun/12/ellen-bialystok-bilingual-brains-more-healthy
The question about trilingualism raises important issues about the practicalities of linking this research to the configuration of the brain rather than social factors. Presumably many people who learn a third language have done so as a choice (although there are many parts of the world where three languages are used).