Let's face it, I'm an idiot. :-)
Missed one improvement I mentioned earlier (the '&& break').

[snip]
is_installed() {
    prog="$1"
    [ -z "$prog" ] && return 1

    ret=1
    oifs="$IFS"
    IFS=:
    for dir in $PATH; do
        [ -z "$dir" ] && continue
        [ -x "$dir/$prog" ] && ret=0 && break
    done

    IFS="$oifs"
    unset oifs
    return "$ret"
}
[snap]

[...]
> So, I'm happy with this now.

No, now I am. :)

Regards, Frank