I could've sworn metronome was a move rather than an item.
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.
Yep, Metronome originally was, and still is, a move. There's also a hold item now which is an actual metronome. As for it's effect, each time after the first you use any damaging move consecutively, it gets a 20% increase to it's base power, the bonus going up to 100%. The bonus is lost whenever the Poke holding the item is switched out or uses a different move.
It's relatively useless since other items do the job better typically. Not to mention that bonus isn't exactly notable until the fourth attack, which is ample time for the opponent to counter whatever you have holding a metronome. If they couldn't by that time, odds are you wouldn't of even needed a hold item to strengthen it's moves.
Interesting that it exists.... such a waste of a potentially good event. Why couldn't they give it something insane like Skull Bash? That would be kinda funny.
Also, the fact that it's a Piplup is also interesting. Did someone in the anime recently obtain one, or something?
But Skull Bash ain't any good...
Speakin' of the anime, I recall seein' on Serebii.net that Meloetta made an anime appearance some time ago. Hopefully an event will show up within a bloody year, I'm mighty curious how useful it'l be competitively.