Had to re-tile a large image, so spent a bit of time converting tile cutter to use NSOperationQueue. After a little playing, I opted for one operation per row, which seems to give the best all-around performance for the widest variety of images. You can download the latest version from here, or pull the source from GitHub.
For those of you having trouble running the application, I've also added i386 to the supported architectures. Because of my use of blocks, Snow Leopard is required to run the app.