iOS: Previously mentioned Transmission already has a pretty nice remote, mobile-friendly web interface, but if you're looking for a native app experience to monitor your torrents, iControlBits for the iPhone will do it with speed and style.
Before you use the app, you'll need to set up the web interface for Transmission on your Mac. We've discussed how to do this in detail before; just note that now there is no need for Clutch—all the settings are in Transmission's preferences, under the "Remote" tab. After setting that up, navigating the setup in iControlBits should be a cinch—just hit the plus sign to add a new server and use the same IP address, port and login info you used in setting up Transmission.
If all goes well, you should be able to monitor and edit your downloads right from your iPhone or iPod touch, even if you're nowhere near your home computer. The app is fast, sleek and much more responsive than the webapp, so it's a nice download for those that want to keep a close eye on their torrents. Right now, though, you can't add new torrents from iControlBits; you'll still have to do that from the webapp. iControlBits' developer hopes to add it soon, though, so keep an eye out—that is, if Apple doesn't pull iControlBits from the App Store first (as they've been known to do with BitTorrent-related apps).
iControlBits is a $2.49 download for iOS.