"Use iCloud" Pages/Numbers option won't stay turned on
I have a strange problem with iCloud synching and the iWorks apps (Pages, Numbers) on my iPhone 4. The problem is this: if I turn on "Use iCloud" on the app settings, then go into Pages or Numbers, I don't see the documents that I have stored in iCloud. Similarly, the documents in my iCloud don't sync to the iPhone. If I go back into Settings->Pages or Settings->Numbers, the "Use iCloud" option has switched from "on" to "off". I can do this any number of times with the same result. To summarize:
1. Settings -> iCloud -> Documents & Data is turned "on".
2. Go to Settings -> Pages.
3. Turn "Use iCloud" to "on".
4. Go to Pages. Observe that no sync to iCloud has taken place.
5. Go back to Settings -> Pages
6. Observe that "Use iCloud" has reverted to "off".
It works fine on my iPad with the same Apple ID so I'm pretty sure it's not my account. I've tried resetting the phone and restoring from an iCloud backup but that hasn't helped. I'm definitely running the latest version of Pages and Numbers (and the version numbers are the same as those on the iPad which works). Settings -> iCloud -> Documents & Data is turned on as well.
Has anyone seen similar behavior? Any idea how to fix it?
iPhone 4, iOS 4.3.2, GSM