Why are so many people still unaware that you can move files to your iPad via iTunes. And 3rd party apps can gain permission to use these files. GoodReader for example can read PDFs, .txt etc that are put onto the iPad via iTunes. If you're wondering, you go to the Apps tab and at the bottom is the File Sharing area.
It's on a per app basis though. I can't take a document out of GoodReader and open it in Pages to edit.
A centralized store for documents would be ideal, much like 'camera roll' for pictures.