Though Apple did not document yet if and how the iPad supports the startup image tag for web applications:
<link rel=”apple-touch-startup-image” href=”/startup.png”>
After some trials and errors I found how to make it work. You just need to make sure that your image is set to size: 1004*768 portrait.
Note that it is 1004 and not 1024. They did the same with the iPhone: 460 instead 480..
Hope you will find this useful.