As I said this is only poseted quickly so the explanation may be a bit glossed over. However it works somewhat like the voltage tripler of a television set. Where AC is passed through a series of diodes and capacitors progressively adding the voltage at each stage up the ladder. In this case there is only one stage effecively doubleing the voltage. But more to the point, it's main advantage is that you can derive a split supply without the need for a centre tapped transformer at all. The trade off however is that you loose current. Still I've managed to draw 800ma per side through one of these arrangements. And the big advantage is that you can do it with a bunch of ordinary rectifier diodes and a bunch of electrolytic caps.
Although the original poster required a 15 volt supply and this circuit shows only + / - 12, there should be enough voltage generated across the pre-regulated side to deliver the required 17 volts for regulation. If not you have two choices. You can either add a secon set of diodes and capacitors or you can use a higher transformer voltage. In the case of the original poster's problem, he had a 24 volt centre tapped transformer so it would be reasonable to assume he could just use it as a 24 volt single winding type.
Note that this will only work with AC. It is ideal for use with a plug pack/wall wort.
I hope this is for some help to people. It's a circuit I tend to use again and again and is probably worthy of reference.
Be absolutely ICebox.