Brian nailed ot on the head... I couldn't have said it better myself!
One of the interesting options I noticed on the BBM 5.0 release was the option to "PIN Contact" when you have a convo with a contact open ? Seeing as BBM is Blackberry to Blackberry is this just a carryover from previous apps ? Seems kind of redundant to me, or is there an other feature ? Perhaps if the other person does not have BBM ? But then how would they show up in ur BBM Contacts ?