It depends if I have gotten over the person completely (and I admit to only really having been infatuated, not in real love). Once I am over someone, that is it. The door is shut. This is especially true if they have done or said anything to cause me to shut that door. I usually make a real effort to move past it and detach, and while it may take time, I am always successful.
In the case of an ex-friend I loved like a sister, I hold some small curiosity for her, but no real feeling. It took awhile to let the friendship die inside me, but like I said, once the feeling is gone, it's gone. I don't hold onto futile feelings forever.