I noticed when working with iPhone and iPad keyboards that they are quite a significantly different colour:
Is there any particular reason for this?
The most important difference seems to be the contrast between the key background and the letters. Note also that the font on the iPhone keyboard is bold while the iPad's is not.
The iPhone is much smaller and likely to be used outdoors so the extremely bright off-white shade of the keys makes sense. Black-on-White is ideal for outdoors, though the iPad's Black-on-Grey looks more elegant and frankly more real.
The background color was probably picked just to accent the key colors on both platforms; the iPhone's colors are more bright and artificial, so the brighter bluish gray makes sense. The iPad's grey keys work well on the warmer but darker looking grey background it uses.
Note the default iPhone UI is very bluish gray and grayish blue1:
The default iPad UI elements are less monochromatic2:
The blue fits in with the default iPhone UI elements quite well with it's slightly washed out blue-gray. Consistency seems to have been more important in the iPhone UI defaults.
Higher contrast to compensate for smaller size and smaller gutter space?
Side by side in (more or less) real-life proportions it makes more sense to me: