From the unity iphone manual:

You can save required files to a Documents folder next to your game's Data folder.

public static string GetiPhoneDocumentsPath () 
        { 
                // Your game has read+write access to /var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/Documents 
                // Application.dataPath returns              
                // /var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/myappname.app/Data 
                // Strip "/Data" from path 
                string path = Application.dataPath.Substring (0, Application.dataPath.Length - 5); 
                // Strip application name 
                path = path.Substring(0, path.LastIndexOf('/'));  
                return path + "/Documents"; 
        }

You can Cache downloaded asset bundle using .NET file API and for reuse it in the future by loading it via WWW class

    // Code designed for caching on iPhone, cachedAssetBundle path must be different when running in Editor
    // See code snippet above for getting the path to your Documents folder
    private var cachedAssetBundle : "path to your Documents folder" + "/savedassetbundle.assetbundle"; 
    var cache = new System.IO.FileStream(cachedAssetBundle, System.IO.FileMode.Create);
    cache.Write(download.bytes, 0, download.bytes.Length);
    cache.Close();
    Debug.Log("Cache saved: " + cachedAssetBundle);

In short, yes you can save files on the iphone with the System.IO.FileStream