The first is an app called iTransmission. It’s jailbreak-only, which means you’ll have to head over to the InsanelyI Cydia repo (repo.insanelyi.com) to get it. The app is currently in alpha, so it might be a little buggy, but it’s free and fully featured.
Your second option is an upcoming web app called NowStream. It lets you grab a torrent link, copy it into the web browser on your iPhone, and stream the contents of the torrent instantly on your advice. Meaning, yes, you can stream torrents to your iPhone without a jailbreak using this app! Unfortunately, NowStream is still sorting out some legalities at the moment, but you can sign up at their website to get notified as soon as they launch. [Cult of Mac; NowStream]