Sites on Multilingualism are becoming more and more common. The Ministry of Education of New Zealand’s LEAP  (Language Enhancing the Achievement of Pasifika) site has the following key points and link. Check it out at: http://leap.tki.org.nz/Is-bilingualism-a-problem.

• There is still a widespread view that bilingualism is disadvantageous to learning.
• Early research into bilingualism tended to reinforce this negative view, suggesting that monolingual students performed better than bilingual students in a range of cognitive and learning tasks.
• A ‘container’ view of the brain, where it was thought that learning another language impacted negatively on or ‘pushed out’ the existing language, reinforced the perception of bilingualism as a problem. It has since been found that this is not the way the brain works; rather, languages are linked in the brain by a central processing unit, meaning that people can easily learn two (or more) languages.
• Subsequent bilingual research has also discredited the early bilingual research, consistently demonstrating that bilingual people have clear cognitive advantages over monolinguals. (See also the inquiry Is bilingualism an advantage? .)