If I were a conspiracy theorist, I'd assume it was because they ended up scrapping an announcement about a special-edition band-themed iPod nano. But for the life of me, I can't think of a band that sings about anything yellow...
My pragmatic side thinks they forgot to turn the "Apple Exclusive" layer off in Photoshop when they were saving the yellow nano image file.
Doesn't Coldplay have a song about yellow? Hmmm...