do I have to have iWork to store documents on iCloud?
Do I need to purchase the iWork apps (Pages, Keynote, Numbers) in order to store documents on iCloud? I didn't need to on MobileMe. It certainly looks to be the case.
iCloud
you need to have these mobile apps on your iOS5 device in order to be able to use the iwork-web-app storage/upload system in www.icloud.com, yes.
Iwork on a Mac is not necessarily needed - because the OSX versions have no iCloud features at all. You could as well use OpenOffice to create Microsoft compatible doc/xls and ppt files and upload these - the iOS apps will open them all as long as the format is Microsoft OR iwork09.