Stu Maschwitz at ProLost has written a post about his experience with the iPad for the past week. In particular he looks at viewing PDFs (of scripts) as well as writing scripts on the iPad.
He seems to like the document reader ReaddleDocs for viewing PDFs.
He also talks about script writing tools; there's a couple of iPhone script tools, but there's no iPad specific application, yet. But there is always Pages, and Joke and Biagio have created a template that works in Pages.
Note that if you have the Mac version of Pages, it comes with a script format template and I have transferred that across to the iPad and it seems to work okay.