If it is important for you to know the status of your potential partner BEFORE you know them well enough to know whether or not you want to sleep with, ASK THEM YOURSELF. Grow a pair & stop making it all the potential-positive person's responsibility to inform you. If you put your preferences in your profile, do be respectful (ie. using "hiv- seeking the same" vs. "d/d free" or "clean").
If you are asked directly, a direct honest answer, however difficult, is appropriate.
HOWEVER, people should understand why an hiv+ person might not volunteer the information on the first date. The hiv- (or presumed negative) person who only wants to date hiv- people might see it as a waste of their time, but consider this: They are going on a first date with someone they barely know, let alone trust, therefore they have no way of gauging how discrete you will be with the matter (and let's face it there are buttholes out there who would make things difficult for someone with the disease -- spreading gossip if nothing else).
If during the date it becomes apparent that this is not a good match, the issue doesn't need to be broached, and the hiv+ person's privacy is protected. However, if it looks like there is a connection there, then I would contact them the day after the date, tell them you enjoyed yourself and find out if they were interested in meeting again. If they stay yes, then let them know there is something you need to tell them before it goes any further, present them with the details and then ask them how comfortable they are with that. Regardless of their answer, thank them for their honesty and let them know you'd appreciate them being discrete with this information.