PO4 does not leech, it just does not get bound anymore - the result would be the same as if it did leech... it rises. Once phosphate binds, it is bound until the aragonite is melted or some bacteria unbinds it to immediate consume. Some organics on the rock might die resulting in some increase phosphate, but this is not true phosphate leeching.
This post sums it up with the sandbed, which is the key:
http://www.reefcentral.com/forums/showpost.php?p=21765620&postcount=86
You can read that whole thread if you want for more info. I got confused in it for a while, so ignore my posts.
Basically, it breaks down like this:
1). Have aragonite
2). Let your tank cycle the whole way (nitrate cycle too) - can take 4-6 months
3). Don't disturb the equilibrium by using GFO, vinegar, etc.
4). Keep the sand clean so that the bacteria can be in control - replace the sand or clean it slowly but methodically
5). Clogged up sand will starve out the bacteria and once all of the aragonite is bound up with phosphate, you will have phosphate issues... so don't let it get clogged up. Old tank syndrome is actually "clogged up sandbed" syndrome.
It works and is sustainable, but it takes some effort to clean or replace. The best thing is that the bacteria count will rise/fall as needed to support your load as it grows/shrinks.
If you want to see this working, feel free to stop by. I have 2 tanks with pretty much just waveboxes, skimmers, heaters a return pump - undetectable N and P, good color and growth.