If you aren’t already aware, the latest version of Transmission for the Mac (1.4+) allows for a web interface which can be controlled from an iPhone.
There’s two ways to use this feature; you can do it the easy way and use it only when connected to the same network as your computer, or you can go the hard way and actually control it form anywhere you want. This raises the question of why you would want to do it from anywhere, but you can if you want.
There’s an amazing tutorial over at SmokingApples.com that you simply have to check out. I warn you though, in order to get the iPhone controlling transmission from anywhere else but your own home network takes some pretty heft networking skills and lack of fear.