Hi, I use Mailbuild (Campaign Monitor's sister service) and I managed to modify your module to work with Mailbuild too. Here's how to do it in case your module users want to know too.

First you need to generate your Mailbuild API and list ID (http://www.mailbuild.com/api/)

Change the following changes in newsletter_checkbox.module

Line 54:
$s = new soapclient('http://yoursite.createsend.com/api/api.asmx');

Line 61:
$rVal = $s->call('Subscriber.Add',$prams,'http://api.createsend.com/api/', 'http://api.createsend.com/api/Subscriber.Add','','','rpc','literal');

Thanks for a really useful module!