Michael Erard had this op-ed piece in Sunday's New York Times questioning the use of the Census Bureau's question as to whether a language other than English is spoken at home really is sufficient to determine whether someone is actually bilingual:
But a moment’s reflection reveals that the bureau’s question about what you speak at home is not equivalent to asking whether you speak more than one language. I have some proficiency in Spanish and was fluent in Mandarin 20 years ago. But when the American Community Survey (an ongoing survey from the Census Bureau) arrived in my mailbox last month, posing that question, I had to answer no, because we speak only English in my home.
I know I’m not alone. There are countless Americans who speak languages other than English outside their homes: not just those of us who have learned other languages in school or through living abroad, but also employers who have learned enough Spanish to speak to their employees; workers in hospitals, clinics, courts and retail stores who have picked up parts of another language to make their jobs easier; soldiers back from Iraq or Afghanistan with some competency in Arabic, Pashto or Dari; third-generation kids studying their heritage language in informal schools on weekends; spouses and partners picking up the language of a loved one’s family; enthusiasts learning languages with computer software like Rosetta Stone. None of the above are identified as bilingual by the Census Bureau’s question.
Ignoring the obvious fact that he doesn't know if anyone cited in the examples does or does not speak it at home, he ignores the basic question of what constitutes multilingualism. I don't believe that speaking a few words of Spanish to someone makes me bilingual. I speak a few phrases in French to some native French speakers I know, but wouldn't say I know French, can still speak enough German to make my way around without having to say "Sprechen Sie Englisch?", but wouldn't be able to read The Tin Drum and the only problems I really have with Spanish is in not confusing it with Portuguese.
But the second language I feel most comfortable with is Portuguese and it's precisely because we speak it at home. When I committed myself to Mércia, I did not believe it was fair to put her in the position of having to translate for me with her family members and friends who did not speak English. Accordingly, I also committed myself to gaining fluency in her language. The cornerstone of that effort was that we speak Portuguese as much as possible at home.
I'm grateful to her for her assistance with this, and while any second language - especially one learned as an adult - is always a work in progress, I'm glad that my conversational ability in her language is solid, my ability to write in Portuguese is good and my only area lagging is reading, primarily because I read quickly in English, but not nearly as fast in Portuguese. I feel that I make up for this by the fact that I often think and dream in Portuguese.
It's precisely because I speak Portuguese at home that I'm fully bilingual.