Come Wrath, mounts and vanity pets will be learned as spells, CM Wryxian has just announced. The items that currently summon pets/mounts, like Sprite Darter Egg or Ashes of Al'ar, will serve to teach us the spells, at which point the item will disappear from your inventory and the spells will be found in your spellbook for all time (bindable to hotkeys like any other spell if you want).
This should make collectors very happy: collect all you can now, and they won't take any space whatsoever once you learn them. Be sure to check out WarcraftPets for help on how to find them (that site's creator was recently honored with an in-game NPC). And everyone will gain at least two more inventory slots that are currently devoted to their ground and flying mounts (except people whose mounts are already summonable, i.e. paladins, warlocks, and druids). Excellent move, Blizz; this is even better than the vanity pet bags people have been asking for.