I think on the surface, they can apper similar, but there are differences. I think volatile has a good example of a difference. One thing, is that they dissociate--but in borderline it's more like they are spacing out. I'm not quite sure how that works with DID. One thing that is similar is that, more often than not, there's been some abuse and/or chaos in their lives. The other day, I googled "borderline personality disorder vs. DID" and quite a lot of stuff came up.

p.s. I have borderline personality disorder, and I've had people think I had DID, saying that I was like two different in the same body, that there was "another part of" me that was an emotional cripple, that I was always changing, asking me "Whatever happened to that good girl I used to know?" or making comments like, "That's not the woman I know," etc.