Great question.  I can't wait to see other responses.

Dude, I'm only American and I have no clue.  I think it depends on where you live, in part.  I think in small-town America, and especially the more politically moderate/conservative parts, exclusivity is more likely to be assumed along the lines of your British example.  City folk, well, especially the big liberal cities (New York, Chicago, LA, San Francisco) I'm not sure if there are /any/ rules.  Different people make different assumptions, but it seems to me promiscuity of women is more likely to be glorified compared to small-towns.  (Though I have heard some city people say they don't feel comfortable dating more than one person at a time.)  I also know a lot of people willing to sleep with multiple people on an ongoing basis.  Those people have a tendency to be shadier when it comes to communicating openly about their behavior though, indicating perhaps there is a /little bit/ of a stigma attached to that?

I think there's plenty of Americans who don't know how to date at all, let alone date multiple people at once.  I don't know if the hook-up culture is to blame, or if it's an older trend...

Facebook is the arbiter of all things social; everyone knows that.

I have very strong feelings about this one. I actually got into an argument with a not-yet-boyfriend about it. We'd been dating a few weeks and seeing each other more days than not. But when he referred to me as his girlfriend, I inadvertently burst out laughing and said, "You're not my boyfriend." He asked if I was dating anybody else; I said no. He asked if I wanted to be dating other people; I said no. Then, of course, he said, "Then how the fuck am I not your boyfriend?" I said, "I wouldn't call if you if my car broke down."

That, ladies and gentlemen, is what makes someone a boyfriend in my book. If the shit hits the fan, would I think to call you? Or am I still putting my best foot forward? If I'm comfortable relying on you and asking you for non-sexy favors, then you're my boyfriend. That doesn't mean that all wooing goes out of the window; it's really more of a mental jump than anything else. But still, if I wouldn't think to call you if my car broke down, you're not my boyfriend.

Color me (or colour me if you prefer) clueless.  When I was growing up you just sort of assumed if you were seeing someone that you were a couple.  Exclusivity was assumed, if not always practiced.  

I think the exclusivity conversation, which is pretty friggin' awkward if you ask me, is an important clause in a new book of rules that I've yet to find a copy of.  

With all the other responsibilities of day to day life though, who really has time to see more than one person, even if you're so inclined?  Who has the energy?  When do you have time for yourself?  Perhaps that's the point though.

Celebrities seem to have their own book of rules outside of the one for us common folk.  They change partners as often as the rest of us change our shoes.  

Kate, I think you hit the nail on the head with that one.

Essentially I think that there is a certain amount of non-exclusivity in the beginning which just annoys the fuck out of me. Because of course I assume he's dating other girls, and if I know for sure, I feel like I'm either competing with them, or he's not looking for anything serious. But of course it's far too early for it to be considered "serious".

Which then brings to question what "serious" entails. To me "serious" entails moving in together or getting married. These days though it seems like "serious" == changing your facebook status to "in a relationship". Which is not serious by any stretch of the imagination.

It baffles me too, and I was born and raised here. Ultimately it seems as though we're dealing with a culture that fears commitment. Perhaps (and I'm taking a leap here) it's to do with the high divorce rates in the U.S.

I think using celebrities to prove a point is in most respects, a poor way to adequately judge the state of the nation. Personally I don't believe 75% of the stuff in those magazines. Paparazzi catch a shot of an actress talking to a guy in a club, maybe sitting very close so they can hear each other through the noise, and suddenly they're "canoodling".

Also, I remember reading something somewhere recently about the secret to Goldie Hawn's and Kurt Russell's long relationship. She lets him cheat.

Perhaps that's the key?

I don't know, Princss.  I'd say serious entails the depth of feelings.  I've had what I'd say were serious relationships where we both knew that marriage was out of the question, despite intensity of feelings... where it was understood that it wasn't going to work in the long haul.  I guess it depends on what you're aiming for.  

i've only had the "let's be exclusive" talk with one person - he brought it up, and i was bored w/dating, so i agreed.  it didn't last long. apparently being bored with the alternative isn't the best basis for a relationship.  for me, in my significant relationships, it's always been assumed that, if you like each other, you won't be dating other people.  

I spent eight years, off and on, doing internet dating, and in my book, there were exactly two indicia for whether something was "serious" or "exclusive" (which in my book were synonyms -- if we're not serious, we're not exclusive).  One is The Conversation:  Are we exclusive or not.  The other is taking down the profile.  If a person's profile is up, then he/she is open to dating others.  

As I think back on it, for the ones that became serious, I'd always go through a period where I *could* see someone else, but chose not to.  That's how I knew the relationship was something I wanted to turn serious.  (But that may be my commitment-phobic idiosyncrasy.)

I do think if you are sleeping together the assumption is that you are exclusive.  At the very least, I think it's immoral to be sleeping with multiple people at once without making it very clear that that is what you are doing, so the other person can make an informed decision about his/her own health and emotional needs.