Yeah, the question is "how fast will pets level"? Currently, if you pick up a pet and go run it through a Heroic instance, the level 77 pet will be getting some good xp from the 80-82 mobs. In Cataclysm, our pets are mostly going to be even-con or one level below the mobs you are fighting, so they won't be getting as much of a boost from "higher level" mobs.

Second, if you are power-leveling a pet now, it's just by killing things. As you level yourself, you will be getting a bunch of experience from quests, which doesn't transfer to your pet. (Far as I know.) Also, for that first level your pets will be getting NOTHING. Since they only gain xp if they are lower than you, while you level from 80-81 your level 80 pets will get no xp. The actual pet-leveling won't start until 81.

It used to take pets about half your level to catch up. Meaning, once you got to 81.5 your pet would finally hit 81. Thus the best you could do was 2 pets at a time. I think it's been sped up since then, but I don't know what the rate is in Cataclsym. So my completely unfounded guess is that you could get 4-6 pets leveled each of your own levels. Once you hit 81, you start leveling your pets and trade them out as each one "catches up." You'll probably manage to keep your 5 "active" pets up to speed with you (equal or one level below), but probably won't have much time to get any stabled pets up to speed.

Once you hit 85 you can get the rest caught up. You could tame a couple new pets and only level them to 79 so you can level them while you go from 80 to 81, but that wouldn't actually help anything. You'll still have to level them from 81 to 85. Instead, if you wait for 85 and then tame pets, they'll automatically jump to 82.