A few days ago, TorrentFreak speculated about how someone might create a bittorrent client for the iPhone. While it seems technically possible (except for that nagging "no SDK" problem, which makes it hard to get code running on there), you might question why it needs to be done - since there are plenty of good legal torrents out there, it just seems easier to get them on your Mac first, and then sync them to your iPhone. Still, I'm sure there are quite a few people out there who want to do torrenting on the fly, so maybe it'll be done eventually.
In the meantime, the P2P blog has a neat tip that lets you almost get bittorrent on your iPhone. Almost every bittorrent client (including Azureus, which is what I use on my Mac) can establish a remote connection via a browser, which the iPhone has. Using a plugin for Azureus (here's one that P2P recommends, and here's another that they say might work better with the iPhone), you can start and stop downloads, and even queue up local torrents.
At this point, the plugin's search function doesn't work (so you must have the torrent sitting on your local box in the first place), but in the future, you'll be able to find and queue a torrent on your iPhone, and then have it ready to go when you get home.