If you are looking for a method to send a completely unique message to each contact, that would potentially be very difficult to accomplish. However, if you simply need to customize small portions of the campaign for each contact, that can definitely be accomplished through the email campaigns that the API is able to create and send.
We allow emails to be created using a special XHTML format that allows you to merge contact data and contact custom fields into the email campaign. Thus you do have the capability to use information such as a contact's name and email address to customize the campaigns you send. If this method is of interest to you, you can find information on Constant Contact's XHTML tags in section 4 of this guide: http://img.constantcontact.com/docs/pdf/AdvancedEditorUserGuide_1010.pdf
If you have any addition questions or if I can clarify, please let me know!