It's always been annoying to be a Hunter in search of a new pet-- you have to either stable or permanently abandon the one you have with you, which leaves you without a pet for a while. And considering that taming new pets is how you learn new pet abilities, it's a pretty important thing to do. So for a long time, Hunters have asked for a way to tame new pets without having to go without their old ones for a while. And apparently, that's what Petopia found is now live in the new patch-- when you want to tame a new pet, you need only dismiss (not abandon) the pet you have, and you are able to somehow tame another pet.
If you then go back to the Stable Master, your dismissed pet will have been placed in any stable spots you have open. If you don't have one open, but then buy one, Petopia reports that the dismissed pet will appear in your purchased slot, and that all three pets will be available to you, even after you log in and out. You can even dismiss and tame a pet multiple times-- at one point, Petopia had a total of five pets (two inaccessible from the stable) to juggle at a time.
This a huge feature to be undocumented, which actually leads me, and a few other players, to believe that it's a bug. People who have tried it report that some strange things are happening-- selected pets are showing up in the wrong place and so on (although from what I've seen, no pets have actually been lost yet-- then again, there's always a first time, so go for this at your own risk). It could be, also, that this was a feature Blizzard was building in, but didn't get done in time for the patch, and is being found in its early state by Hunters. In the meantime, now might be the best time to go learn those new pet abilities you've haven't been able to pick up because you didn't want to abandon your three best friends.