The ant completion script that comes with Ubuntu 12.04 seems broken; this is another version which will list available build targets and options, when you press TAB a couple of times.

Put the file ant-completion.bash somewhere it gets sourced, or source it inside a startup file (.profile, .bashrc etc). As an example you can put it inside a folder inside your $HOME directory, if you put it inside $HOME/.bash_completion.d and add this section to one of your startup files:

if [ -d "${HOME}/.bash_completion.d" ]; then
    for f in "$HOME/.bash_completion.d"/*;
    do
        . "$f"
    done
fi

you should hopefully get some completion for ant again, you can of course just source the script as well..

source ant-completion.bash