As I think someone else wrote before me, I think obvious boundaries (for me) are RL details; emails, addresses, towns, and even down to personal social media pages. If I’m having a chill session with a few buddies, I don’t mind getting personal, though. I think, when you’ve been friends for so long, trust is formed, and certain things like “BRB guys, I have to go take a poop!“ or “How old is everyone?“ are no longer risky things to either bring up or ask.
With that said, it also helps with the situation one is in. If I’m hanging out with friends in one of their homes, I’d be a lot more comfortable. Even if the environment isn’t physical, the aspect of the 'home' feels welcoming, RL or not. The same thing goes for the sunset comment Darrius made in his original post, and I wholeheartedly agree. Some moments in Second Life evoke the same feelings one would have in real life.
I digress; my boundaries between SL and RL are still strong. I know which one is which, but that doesn’t stop me from enjoying a sunset, or a talk with a friend on an open prairie, and that’s what I like about SL. I can go somewhere I can’t go in real life due to financial reasons and/or timing.
I hope I answered this correctly, and hope I stayed on topic. It is a complicated question, I think.