Saurik, the developer behind Cydia, Cycorder, and many others, has successfully ported the old MobileFinder version 1.1.4 to iOS 2.0 and above. A similar MobileFinder in the AppStore can also work with files, but does not have access to the root directory, as a result of which, it is not able to harm the iphone file system. Thus, this program is a full-fledged file manager for the phone, with the proviso that it must be used carefully. The only error I encountered is that your settings are not saved when you exit the application. If you opened access to system files, then you will have to re-enter this setting the next time you start the program. The program is installed from Cydia. 
