Hi Dave,
You -could- accept a comment in a custom field (textarea) on your signup form, however, ListMail currently only allows editing of custom data on a single-line "input type=text" field. I plan on providing options for different types of custom fields in future versions.
With "notify when a user subscribes to this list" enabled on the List Settings page you would receive a copy of what the user wrote as a comment, but it would not be easy to reply.
What I recommend is instead of using signup notification, use the included xtra/signup-xtra.php script. This script could send a separate email to your email address appearing to be "from" the client, therefore allowing you to reply easily.
ie:
if($list=='1'){
// comment = user1 (custom field 1)
$subj = "User Comments";
$msg = "$fname $lname wrote:
$user1";
mail('you@example.com',$subj,$msg,"From: \"$fname $lname\" <$email>");
}
Regards