Just a Pages editor
I'll probably never use Keynote or Numbers, but I'd like to use Pages. The Notes app on iOS is pretty lame, and I would like to keep track of dozens of docs if possible. I can already do that on my iDisk, but those docs are read-only. (And Google Docs for mobile didn't allow editing the last time I checked.)
It might be best to have local cache of Pages docs on iOS devices that sync via MobileMe to iDisk and your other iOS devices. That method is already in place, of course. You could edit on the go without a wi-fi or AT&T connection, and have everything automatically sync later.
The problem would be merging different versions of the same doc. Edit a file on your iPhone, then on your iPad, and you might have a conflict. Notes simply asks you which version is the correct version. Pages docs might be more complex, and would require line-by-line merging. Maybe this is why Apple hasn't done iWork for iOS yet.