It's different being in a relationship and not knowing about your condition, as opposed to knowing about the dx and withholding the information.

My feeling is that BP is a path that cannot be walked alone. It will take too much out of you to keep it from your partner. And I also believe there should be no secrets in a relationship: would you not tell your partner if you were dx'd with cancer? Well, I don't think BP should be treated much differently.
I could never keep my dx to myself - my partner needs to be aware. He needs to know about my mania symptoms (If I was prone to it) in order to get help where necessary. He needs to know about my depression, so he can support me, and not trigger me.
I've been dx'd for about 18 months and things are getting better every day