$ cd ~/annex
$ git mv big_file my_cool_big_file
$ mkdir iso
$ git mv debian.iso iso/
$ git commit -m moved

You can use any normal git operations to move files around, or even make copies or delete them.

Notice that, since annexed files are represented by symlinks, the symlink will break when the file is moved into a subdirectory. But, git-annex will fix this up for you when you commit -- it has a pre-commit hook that watches for and corrects broken symlinks.