Code review

I did a quick code review. All of these are (extremely) MINOR points. Fix them as you please.

1. Try not to use tabs. We use 2 spaces.
2. I'd avoid using "simplenews" in output shown to the user/admin. For example, I'd rename the permission administer simplenews to administer newsletter.
3. Also in URLs, I'd use 'newsletter' and not 'simplenews'. It makes them easier to remember/guess.
4. I'd consider to rename the list types-tab to newsletters. Using the term list types makes sense if you are a developer but is less intuitive for regular users. (You have multiple newsletters, and each newsletter has multiple newsletter issues (or issues for short.)
5. We use '-' (dash) in CSS class/ID names, not '_' (underscore).
6. $sn_form .= strlen($email) <= 30 ? $email : substr($email, 0, 29).'...'; will fail when multi-byte UTF-8 characters are present. Remember we are always dealing with UTF-8 strings, not ASCII strings. Use truncate_utf8($mail, 29, FASLE, TRUE);. It's a Drupal function that lives in common.inc.
7. In core we write 'E-mail', not 'Email'.
8. $sn_form = form($sn_form, $method = 'post', ''); looks like a copy-past error. Probably should be $sn_form = form($sn_form).
9. We usually don't glue words together in the URL scheme: newsletterconfirm/remove/$subscription-id would be something like newsletter/confirm-remove/$subscription-id or newsletter/remove/$subscription-id.
10. $message = t('Newsletter "%title" could not be sent to %email.', array('%title'=>check_plain($node->title), '%email'=>$mail->mail)); watchdog('simplenews', $message, WATCHDOG_ERROR);
    For consitency, use theme('placeholder', $foo) with watchdog() and drupal_set_message(). This ensures that messages are themed properly. theme('placeholder') calls check_plain() for you, and makes sure messages are themed properly/consistently.
11. t('Newsletter subscription status confirmation notice for').' '.variable_get('simplenews_from_name', $name_default).' '.t('newsletter.'); is not easy to translate. Better to use a variable %newsletter-name so you don't have to break up the sentence. That gets you translation bonus points.
12. drupal_set_message(t('No addresses were added.'),'status'); You can omit the 'status'. It's the default value.
13. In CVS HEAD there is some timer functionality you can use to replace sn_time(). See timer_start(), timer_read() and timer_start().
14. Substitute $GLOBALS['HTTP_SERVER_VARS']['HTTP_REFERER'] with referer_uri().