Till now there is no approved iPhone or iPad application for torrent download but if you have internet on your iPhone, µTorrent Web is what you should have bookmarked. This is optimized website particularly for iPhone users and if you have the web access enabled in your desktop client, you can manage all the downloads right from there.
Once you have created a login into your client, which is under Options > Preferences > Web, go to the weblogin page from your iPhone and login with the same credentials. Done that you would see uTorrent client like design made just for iPhone users. Happy Downloading!!
Related : How to limit bandwidth usage in uTorrent [ and Save Money ]