All addons listed here previously now support BreedIDs by default - either by routing their calculations through Battle Pet BreedID or by using their own method (usually libpetbreedinfo).
Battle Pet BreedID can also be used as a library-type addon that provides two functions that give the BreedID of a pet. This can be then used by other addons to display the BreedID or some associated icon or graphic based on the BreedID, and give players information about how the pets they see in their journal or in other situations in battle.
GetBreedID_Journal(nPetID) is the first function, which is used for viewing pets in the journal. As you can see, it requires an argument of the PetID relative to your journal - not the speciesID of the pet.
GetBreedID_Battle(self) is the second function, used for when the pets are in a battle. The required argument is self, which is required for most of the pet battle functions, and should be somewhere in your addon already. If not, see the official api examples.