Transmission Client for iPhone

There was, called DriveTrain, but it got rejected by Apple due to "copyright issues" i.e. the potential to download illegal torrents. Shame really, it looked a really polished app.

Linky.

I think you can remotely monitor Transmission through a web browser, though...

Actually there is, check iControlBits.
Looks great. I've contact developer, adding files comming in next ver.