Thanks! It's kind of you to answer me and your messages are really helpful.
I only differentiated between strict OPOL and what I propose to do myself because I know a couple of families where the parents do carry on all conversations in two languages, at least in front of their kids. It's quite doable and you get used to it within a few minutes but I agree that it might send the wrong message. You wouldn't want your kids replying in one language when you spoke to them in another (although for many parents I've met, this is the daily routine--the kid constantly slips into the dominant language or even speaks it exclusively while the parent continues to carry on his side of the conversation in his own language in a vain attempt to get things back on track...). Also, in these families the parents can understand each other's language but are not really bilingual themselves so don't have much of a choice, or else they are trying to teach two minority languages at once it is especially important that the two get equal time to compete with the outside language. I think we'll try my second idea after all.
We, too, used to switch back and forth depending on the country we were in but now English has become so dominant in our relationship that I feel strange and uncomfortable if my husband happens to speak to me in French when we're alone. So now we're just another American couple when traveling in the US, but I think he enjoys blending in there and tricking everyone.
It is lots of fun having your own language and, surprisingly, English does work as a secret code here in Paris (and especially at my in-laws' house, too!). It oughtn't because in theory all French children study it for many years in school but it is usually badly taught, I'm afraid, and if we speak quietly and at a normal rhythm we can be confident that we're incomprehensible to most people around us. We are still careful but conversations between husband and wife are already so full of familiar abbreviations and allusions, whatever the actual language, that we're pretty safe. We have heard some funny or rude comments about ourselves, our clothes or appearance, or about Americans, Australians or British people in general, whatever they take us for, from people sitting right next to us in the train or restaurant who think we must not understand French if we are speaking anything else. Quite horrible, sometimes. Laughing at them seems the best response. I would be really upset if my kids were affected by that kind of attitude, though.
For a huge and diverse metropolis, this is a strongly monolingual environment. National identity here is tied to the French language (and to food!). Families who succeed in raising bilingual kids seem to have to fight against the public education system and all sorts of hostile attitudes. In the university English dept. where I teach, I've had a surprising number of students from bilingual families, one French and one Anglophone parent, who claim to be bilingual themselves but whose language skills are catastrophic. No one makes them take the standard language entrance test because they have such family credentials. French is their only real language, and when they speak or write English (just like any other foreign language they half-learned in school) it is a dreadful word-for-word translation of the French in their heads. Several of them have very strong French accents in English, full of typical pronunciation mistakes but with an occasional perfect English sound thrown in--one of the few words they actually learned as children? Others pronounce well but only speak baby talk, familiar and limited vocabulary and diction that are not appropriate to academic or professional situations. Others understand but cannot speak at all--they answer my English questions in French. None of them can write decently in English and they tend to skim for a general meaning when they read it. Many of my students who only learned English in school have a far better level than the ones who are bilingual from childhood.
I have had a very few exceptions--truly bilingual students who could have passed for a native of either country, who could think directly in the other language, had vocabulary and writing skills on a level with their French, etc. I am intensely curious about all these different cases, of course, so I always question them all about their linguistic upbringing. The truly bilingual ones all seem to have either gone to bilingual or English private schools here, spent some years living and going to school in an English-speaking country or had two English-speaking parents who never spoke French to them at home and made huge efforts to teach them to read and write in their non-school language and to give them all the breadth of vocabulary of a native speaker.
Of course, there are different levels of bilingualism, all worth something in their own way, and it's good to teach your children whatever you can of your own language. I don't mean to say that the efforts of all these parents were wasted, but they did not result in a usable language or in an enriched cultural identity for their kids. After all this, I feel much less confident that one parent alone can truly transmit a language and the culture that has to back it up to make it mean anything.
Sorry to be so long, for anyone who has had the patience to read all this. It's turned into an entire article. My own kids are just kicks in my stomach and fuzzy ultrasound pictures for the moment, so I really ought to relax for now and not worry about what language will dominate for them in 20 years.