There are two ways I have been able to this. The first is to use multiple constant contact accounts and have multiple API's on your website sending to different accounts. Just make different welcome emails on each account. The other option if you are using WordPress is to use Katz Web Services Constant Contact API along with Contact Form 7. Katz put out an additional plugin called "Contact Form 7 & Constant Contact" that you can use to make seperate forms on your WordPress site and assign them to specific list. There is an Email Auto responder functionality in Contact Form 7, so you can tailor specific welcome letters. Just remove the welcome letter from Constant Contact and you are all good. Hope that helps.
... View more