The Buttons
Check out CodePadawan: iPhone Glossy Buttons for programmatically created calculator buttons.
However, I would suggest using PNG images for the button backgrounds. You can extract the normal button backgrounds (I didn't find selected button background.) from iPhone UI Vector Elements (linked to from iPhone and iPad Development GUI Kits, Stencils and Icons) with Adobe Illustrator by making a selection, copy, new, paste, Object > Artboards > Fit to Selected Art > Save for Web.
CalculatorMemoryButtonBackgroundNormal.png & CalculatorMemoryButtonBackgroundNormal@2x.png

CalculatorOperatorButtonBackgroundNormal.png & CalculatorOperatorButtonBackgroundNormal@2x.png

CalculatorDigitButtonBackgroundNormal.png & CalculatorDigitButtonBackgroundNormal@2x.png

CalculatorEqualButtonBackgroundNormal.png & CalculatorEqualButtonBackgroundNormal@2x.png

The Implementation