I think I should have said this at the very beginning of the thread, but when I talk about being "closeted" and being "out", I don't talk about any particular absolutes. First of all, unless you are visibly, undeniably gay, you will never be ENTIRELY out, because every new person will - as you said - assume you're straight, or at least might assume so. However, to me, there is a line, somewhere - and I can't really point at it, because I think it is in different places with different people - before which I'd say you were "closeted", and after which - "out".
To me, it mostly comes down to "how do you live your everyday life". If someone is out to his family and friends, and "on the street", so to speak, yet isn't out in his workplace, to me that would still be "out". Or as an earlier example here, not being out to family that lives far away and comes from a homophobic culture, while being out to everyone in the place you live.
But if you are out to your mom and your best friend from high school, yet you have cover stories for everyone else in your life, that is not "out", that's "closeted".
Basically, I guess I'd put it like this - if you live your life being worried and/or taking extra precautions about being found out, then you're closeted in my book.