Check out this Terry White tutorial on publishing for iDevices. The only thing I'd add to his advice is regarding the iPad. Instead of using his recommendation of 1024x768 at 72ppi, I'd recommend exporting at 2048x1536 at 132ppi. 132 is the resolution of the iPad screen and doubling the pixel dimensions will give you some room to magnify the image. I can't confirm that this is optimal for iPhone 4, but it works well for iPhone 3.