Late 2010, I'm hoping for and expecting until there's an official announcement. Delays are possible, but Miyamoto is hoping for a late 2010 release.
And why not release Mario, Metroid and Zelda in the same year? Mario could be Q1/Q2, Metroid Q2/Q3 (depending on Galaxy 2 release date) and Zelda in Q4, with smaller titles like Sin and Punishment 2, Monado and Endless Ocean 2 in between. If the most titles Nintendo have released in one year for one format is 12(is that correct?), why can't they release 6, or probably more in the same year, with 3 of those being big IP's?
In my opinion, it's exactly the kind of big software push that Nintendo need next year. They've still got stuff like F Zero, Starfox, Kirby, Pikmin, Wave Race to use, Wii Vitality, and rumoured revivals like Pilotwings and Kid Icarus. Plus Retro's new title. Any of those could be unveiled for 2010, or 2011. They don't need to spread their first party titles too thinly.