i don't know that it's a behavior that is exclusive to bpd. i think it's a very vivid form of daydreaming and it's not really a mental illness issue, since it's not like one with schizophrenia where you can't tell the difference between reality and fantasy. Sure you get lost in these daydreams and if left unchecked I think it would become a real hindrance in your real life but it doesn't sound like a huge issue to me. I mean only you know if it's a real problem but it sure sounds like a pretty good coping mechanism to me. Some of the most famous and successful people were daydreamers I believe, although i can't remember which ones I've read.

The thing is, it's an escape. As long as you keep it in check you'll be alright I have my own escapes and without control I can lose myself completely in them. They always come in forms of obsessions. Diving head first into an interest of some sort. Gaming has been one that was bad for awhile - I would lose my sense of time and place and get completely immersed in my games. Now it's my art. I can be so involved in my art concentrating that all around me melts away. People have been known to get very frustrated when I'm lost in my obsessions. I almost don't hear anything and I'm hard to reach in this mode.

I think it's very similar to having a fantasy that you go to. As long as you're not talking about total dissociation, you're probably ok.