My source directories tend to collect cruft, and it can be a pain to separate the real ack hits from the crufty ones. I am ashamed to say how long it took me to think of the following:

function ackx {

    if [ -f MANIFEST ];

    then ack "$@" `sed 's/[[:space:]].*//' MANIFEST`;

    else ack "$@";

    fi

}

Yes, this could equally well have been a small Perl script involving ExtUtils::Manifest…