iOS 5 for iPhone secretly contains iPad calendar features?
iOS 5 comes in different "flavors" for each device.  iOS is stored on the computer you sync to as an .ipsw file.  These files are located here:

~/Library/iTunes/iPod Software Updates/

~/Library/iTunes/iPad Software Updates/

~/Library/iTunes/iPhone Software Updates/

Each device's "flavor" of iOS is optimized and only contains the code needed for the device it pertains to.