My pet theory is that there is nothing magical about how babies pick up language. Stick me in Spain for two or three years with nothing to do except learn the language (not even looking after myself) and with every one around willing to spend inordinate amounts of time working on simple words and forgiving of all my mistakes, and I reckon I could finally crack the language and get far more fluent than a newborn.
From this TED video, Patricia Kuhl might disagree with me. She suggests that somewhere between 9 and 12 months, babies lose the ability to easily distinguish sounds that are not part of the languages they are regularly exposed to. Her experiments suggest that it is not just about hearing the sounds but having interaction with a real live person speaking the tongue or tongues involved.
I still think I could probably beat a baby at learning a language given a fair competition but, until I can prove that, it sounds like parents from different language backgrounds may have an advantage in giving their children a head start with languages and those without that linguistic diversity between them could do well to cultivate friends from other cultures.