Got this from off a Nintendo news feed which some may find useful. In short it's stated that they can't be caught in the wild.
Pichu: Hatch 2km Egg
Cleffa: Hatch 2km Egg
Igglybuff: Hatch 2km Egg
Togepi: Hatch 5km Egg
Togetic: Evolve Togepi (50 Togepi candy required)
Smoochum: Hatch 10km Egg
Elekid: Hatch 10km Egg
Magby: Hatch 10km Egg
Pichu: Hatch 2km Egg
Cleffa: Hatch 2km Egg
Igglybuff: Hatch 2km Egg
Togepi: Hatch 5km Egg
Togetic: Evolve Togepi (50 Togepi candy required)
Smoochum: Hatch 10km Egg
Elekid: Hatch 10km Egg
Magby: Hatch 10km Egg