Ah, the hidden city question... How many times per year do we get this questions and how many times do we get the "your itinerary will be cancelled" bit.

I think it's valid to point out that there is a possibility that the return segment could be cancelled, but to go through an entire diatribe about the "reconcile process" in hidden rooms somewhere is silly. Although the airlines may do that, I believe they rarely go into no-shows' records to delete follow-on segments. And even if they do it would not take much on the passengers part to convince personnel that you had indeed flown the segment and want the reservation restored. (I know from what I speak)

I have a fair bit of experience doing this with several major airlines and have never had follow-on segments cancelled, or not been able to restore the record by a phone call. The airlines simply don't have the manpower to keep up with it.

I would, however, heed Jetsetter's advice about not doing this with impunity. The rarer the occasion the better. But otherwise go ahead, you should have no problem, I never do.