"Rhythm Heaven could be out in North America on April 5th, according to listings on the websites of Amazon and Best Buy. Gamefly lists the 7th, but their release dates are uniformly a couple of days behind the official one. Nintendo has yet to specify the release date for the WarioWare team's rhythm microgame collection beyond the "first half" of the year. At least Amazon moved their date up from November!
Siliconera notes that Nintendo has scheduled one first-party DS game for every month in the first quarter: Personal Trainer: Math in January, Fire Emblem in February, and Pokémon Platinum in March. Rhythm Heaven could be their one game in April! It could also be the "launch game" for the DSi, also rumored to come out in April."