I haven't monkeyed around with files outside the app sandbox so I don't know what the permissions are for those files. You'll have to try it yourself. Having said that, I'm pretty sure that Apple won't allow an app on the app store that displays info from parts of the file system outside the app's sandbox.
The restrictions to the sandbox are more than just unix file permissions, I think. There is a mobileuser user and root. There are probably some other user accounts but they are all some form of root or not mobileuser. Normally a user could see all the files that are owned by that user. So under a standard unix file system set up when an app is running it should be able to inspect other folders than its own, but this doesn't work. So the file system has other restrictions besides the standard unix file permissions restrictions, I think.