Now that I’ve set this up, I can’t believe that it’s taken me so long to do it. A bit of googling turned up this blog post, which contained a link to this post in the comments. Since I’m using Homebrew instead of MacPorts, I had to tweak the instructions a bit:

• Install bash_completion:

  brew install bash_completion

• Next, add the following to the bottom of your .bashrc or .profile:

  if [ -f `brew --prefix`/etc/bash_completion ]; then
. `brew --prefix`/etc/bash_completion
fi
complete -C /usr/share/ant/bin/complete-ant-cmd.pl ant