Gotchas

If you try to override template_preprocess_user_profile(&$variables)  with phptemplate_preprocess_user_profile(&$variables) the function will NOT be called unless you copy /user-profile.tpl.php in your template directory.

Not 'taking' login and no error message.  If you have restored a whole site check the $cookie_domain entry in settings.php.  If it is not the same as the URL your browser is denying cookies for the domain.

For some reason most of the times the cck data is returned as a serialized string.
When theming you want to unserialize the data so you can access the individual fields.
if(!is_array($item['data'])) $item['data'] = (array) unserialize($item['data']);

The user has "edit any page content" permission but can't see the edit tab.
Check if the user's role can use the page Input format in admin/settings/filters

In Drupal 6 you added a template in your theme directory but you see no change.
Empty the cache since all the templates are cached in the theme registry. Use the develop module to check if the newly added template is in the Theme registry