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
![calculator memory button background normal retina]()
CalculatorOperatorButtonBackgroundNormal.png & CalculatorOperatorButtonBackgroundNormal@2x.png
![calculator operator button background normal retina]()
CalculatorDigitButtonBackgroundNormal.png & CalculatorDigitButtonBackgroundNormal@2x.png
![calculator digit button background normal retina]()
CalculatorEqualButtonBackgroundNormal.png & CalculatorEqualButtonBackgroundNormal@2x.png
![calculator equal button background normal retina]()
The Implementation