BW2; and only BW2; will have a unique Piplup obtainable via wi-fi event. The details are as follows for the Japanese events, and the English details will replace these once announced.
Piplup ♂: Level 15
Hold Item: Metronome
Considering Sing is the worst Sleep-inducing move in the game with it's appalling 55% accuracy, there ain't no reason you'd want this thing for any other point but collecting event Pokes. Although, 'less you can get a Metronome in Join Avenue or some other new way in BW2, wanting this not-so-useful item might be motivation enough for gettin' this Piplup.