First, I think that Spirit Tracks will take much shorter to develop than PH. The Engine is already there. Remember Majora's Mask - it was the same situation, the game was released just 2 years after OoT.
Second, I could imagine that Aonuma doesn't direct ST. This could be the perfect opportunity to let someone else take over the director's chair. Just like Miyamoto did with Aonuma on the second N64 installment, MM (I once read in an interview that Miyamoto retook direction of OoT when the team struggled to finish the game).
So that means that the new Zelda DS doesn't necessarily keep Zelda Wii from getting finished.
On the other hand, I don't really believe they would play the Zelda card twice the same year... I'd guess: a summer release of ST, plus an announcement of Zelda Wii at E3, release next year.
And for the holidays... maybe N will actually release Mario Galaxy 2. They'd be stupid not to, the engine has already been developed, and there still are endless possibilities for level designs.
Currently playing: NSMB (Wii)
Waiting for: Super Mario Galaxy 2 (Wii), The Last Story (Wii), Golden Sun (DS), Portal 2 (Wii? or OSX), Metroid: Other M (Wii),
... and of course Zelda (Wii)