iOS: Starting BitTorrent downloads remotely using Dropbox is a handy trick, and now it's possible in iOS. All it takes is an app called Drop It, a Dropbox account, and the torrent client of your choice.
Using any torrent client that supports watch folders—Transmission for OS X in this case—set it to automatically scan your Dropbox folder for incoming torrent files. With your torrent client running in the background, it will automatically download any new torrent files added to your Dropbox folder.
To remotely send files from your iPhone or iOS device, triggering the torrent to download on your PC or Mac, use iOS application Drop It. By pasting the torrent file's URL into Drop It, it will automatically download the file to your Dropbox folder. There, your torrent client can pick it up and have it downloaded, waiting for you.
In case you already own it, iOS application GoodReader works as a great substitute for Drop It. Thanks for the tip Shawn.
Drop It ($1.19) [iTunes App Store]
GoodReader ($3.99) [iTunes App Store]