Fellow fansite creator Mania (of Petopia and Warcraft Mounts fame) recently contacted me about an item she discovered while testing the beta. Here's Mania's report:
Junior Apothecary Lawrence is an undead NPC in Venomspite, Dragonblight. He doesn't seem to have any quests for me, but when I talk to him, he does have a dialog option. He wanted me to test his Experimental Mixture on a non-combat pet. When I used it on Whiskers the Rat, he turned into a green blob for two minutes!
I thought this might be a random effect, but so far all I've gotten was a green blob. The mixture lasts 30 minutes and you can only carry one at a time, but it appears that you can talk to Lawrence as often as you like to get more.
Mania was also kind enough to include a few screenshots, which I linked above. Thanks, Mania! Yet another nifty pet-consumable to enjoy in WotLK (even if it is only temporary).