Maybe it's a good idea to sort of ease into it progressively.. talk about it in more generally at first and share more information as the relationship progresses.
Although I've been dating for a while and have told people right off the bat and people actually tend to be more open and understanding and some even familiar with mental illness.. either they've experienced some themselves or have friends/relatives who have it. Although I'm taking baby steps with the guy I'm seeing now... as of late I've been sort in the mind set that "I'm perfectly normal and have issues; up's and down's just like anyone else" I mean I really think most, if not all people have their moments and experience at least depression at some point in their life.. and yeah my issues might be a little more severe in comparison but it's not like I'm totally bat ***** crazy and this is coming from the person with BPD.