Please, consider merging functionality with inactive_user module.

Account reminder: Account reminder is a simple module that will resend the welcome email to users who have registered with the site but have not yet logged in.

Inactive user (http://drupal.org/project/inactive_user): The inactive_user module provides Drupal administrators with a way to automatically manage inactive user accounts. This module has two goals: to help keep users coming back to your site by reminding them when they've been away for a configurable period of time, and to cleanup unused accounts.

AR is only focused on users not logged in, and IU does take care only of users already logged in, but in both cases, the UI, the logic and tracking system is the same. I think it is a good chance to have a single (and very powerfull) reminder module.