For web apps you can set the startup screen with apple-touch-startup-image
What about for iPads?
Portrait image needs to be 768x1004 (note: 1004, not 1024, 20px is for status bar), PNG is the preferred file format.
Landscape image needs to be 1024x748 (note: 748, again 20px for status bar). This image then needs to be rotated 90 degrees clockwise, end result is 748x1024.
Link tags in header need to be as follows:
For the new Retina iPad (if you don't add these, it will use the above with pixel doubling):
Previous answers (including the accepted answer) at time of posting this didn't work with my testing.