There were some vague rumors some time ago that the hunter hero class would gain a second pet. Blizzard has said that hero classes were just a concept, and they have no real plans for them. So... no, hunters won't be getting a second pet slot. There will be new pet types in BC though... Warp Stalkers, Dragonhawks, Ravagers, Spore Bats, Nether Rays...
Petopia has all the details (click on "The Burning Crusade closed beta pet info," along the left side).