It's a metronome
The DSi wasn't intended just for games; part of the meaning of the "i" is in reference to using it as a "personal tool," according to some Nintendo rep who may or may not have been full of it. We've already seen some fairly useless applications hit the service already, lending credence to this, so it makes a weird sort of sense for Nintendo to roll out..
About The Game
The Nintendo DSi Metronome software can be used to help keep tempo –when playing an instrument, for example.
It creates a rhythm by playing sounds based on beat and tempo settings, both of which allow a wide range of adjustments. Choose from three metronome designs (including one based on Nintendo's Game & Watch™: Ball title) and record your own sounds for the metronome to use. Or, if you want to take a break, enjoy a unique minigame: Donkey Kong™ Metronome, in which you make sounds into the microphone to match the beat and have Mario™ jump over Donkey Kong's barrels.