Using Fedora/Ubuntu, how can I do it?
You need patched kernel, losetup and mount. The package is usually called util-linux, you can get the patches from here. If you don't want to boot from a loop-aes device it's really simple:
If you want to encrypt the root partition then I recommend reading the extensive documentation. Basically you will need to create an initramfs and store it on an unencrypted boot partition.
You can store the keyfile.gpg (and the boot partition if you decide to encrypt the root) on a removable USB device.