Zoho has been a leader in web document editing. It many ways their products have been ahead of the better know and, presumably, more widely used Google Docs. Their free Zoho Docs iPhone app was just updated as a universal app that now works with (rescales the screen) the iPad.
Unfortunately, Zoho’s updated app is still a read-only viewer for web documents stored by their web service. This, in my opinion, limits its usefulness.
Zoho Docs 2.0 (iTunes App Store)
Google Docs provides limited but still useful editing capability using only the iPad’s Safari mobile web browser. iPad users who need to work with a variety of document types may want to consider evaluating GoodReader for iPad which provides good viewing capabilities and the ability to annotate (but not edit) documents.
GoodReader for iPad ($4.99 in the iTunes App Store)