iCab Mobile can upload to most sites, but you have to use the Open In feature to move the documents from it's originating app to iCab Mobile first. You can not do that with Numbers or Pages. They don't have the Open In feature. There are other ways to get around it, but only for selected cloud storage solutions.
The other Office apps are somewhat more flexible in what services they can use to store docs in the cloud, but they still won't directly upload to a website.
Because there is no general file system, no web browser or app can select a file not stored in that app's folders for uploading. This is why Safari won't even give you the option. It has no file system for you to choose from. iCab Mobile's developers included a file system in the app, but you have to get the files there before they can be uploaded.
Most people just go with cloud storage like DropBox or Box.net. You can upload files there, then use their public links to share, mail or reference the files in other places. Both offer limited free accounts. DropBox works well with Documents to Go and QuickOffice. The iWorks suite needs an additional pay-for DropDAV account to work both ways with DropBox. iWorks does fine using the WebDav services of Box.net, but the box net account does not support local file syncing with your computer, not for free anyway. You can mount the service as a drive, or use the browser. Not as nice as DropBoxes folder syncing.
To just move files back and forth, and view most of them, GoodReader can access (both ways) several different cloud services, including both DropBox and Box.net. It's also an excellent PDF reader/annotator. Worth getting in it's own right for the large number of things it does.
