I played Atlantica (among other f2p games) which basically had exactly what WoW is adding now, although the game was f2p so it was at least somewhat excusable, I can't believe they have the balls to do this for a p2p game.
Basically what happens is the item will be worth a ton when it first comes out. It won't tank quickly, but it will gradually become worth less and less as the new "shininess" factor wears off.
Then Blizzard will replace the item with a new "more shiny" item. The current shiny item will drop significantly, and the new item will be worth a ton. If Blizzard chooses to discontinue sales of shiny item 1 when shiny item 2 is released, shiny item 1 will increase in price in a week or so, before going back down to average levels. If they keep selling all the shiny items at once the older ones will continue to slowly decline until they reach a fairly low price that will end up as the "Standard" price for any non-new item-mall item.