From iPhone Development Wiki
The iOS Keyboard is very complex but one of the most important elements of iOS.
These are approximate measurements in pixels.
Maybe it should be measured in points?
Portait (iPhone, default English US keyboard)
|Space between 2 keycaps||7.5|
|Space between left edge of screen and Q||7.5|
|Space between shift and Z||7.5|
|Space between delete and M||7.5|
|Space from left edge to A||11.3|
|Space from right edge to L||11.3|
|Space from top edge to first row||7.5|
|Space from bottom edge to last row||2|
Landscape (iPhone, default English US keyboard)
|Keyboard||480 (3" devices) or 568 (4" devices)||162|