Your PB Pokédex says:
This is a list of the Pokemon from the Unova region in Pokemon Black & White. In a similar fashion to the Hoenn games, in Black & White the native Pokedex only contains the 156 new Pokemon from Unova. You can later obtain the National Dex which includes the other 493 Pokemon.
Does that mean after I get the NP I can find a "wild Pikachu?"