Italian psychologists have found that bilingual children are more flexible at learning speech structures than monolinguals. When given the opportunity to simultaneously learn two different regularities, bilingual infants learned both, while monolinguals learned only one of them.

The study also showed that although the acquisition rate between bilingual and monolingual infants when learning speech is about the same, the bilinguals learned approximately twice as much about language in the same given time. We've seen this evidence with Abby. Although her speech appears to be roughly the same as that of her peers at school (vocabulary and elocution) she can express herself equally well in a completely different language with vastly different language rules.

This interesting post summarizes some of these observations. If anything, the Eddie Izzard video on being bilingual is quite funny:

Bilingual children learn language rules more efficiently than monolinguals