If you're using double optin like you say you are, it will already work, at least for the welcome email.
Otherwise you can use conditional content:
http://kb.mailchimp.com/article/how-do-conditional-smart-merge-tags-workSeparate lists (bad form, really), or wait for it to be supported via the API before implementing that.
If you have more questions that don't actually have to do with the API, please bounce them off support using the links in the knowledge base.
jesse