How to reset the theme registry
Update for D7:
drupal_flush_all_caches() should work for just about anything. registry_rebuild() alone did not work in my tests.
The Drupal function to reset the theme registry is drupal_rebuild_theme_registry (D6).
You can add it to your template.php while developing:
Adding it to template.php works, of course, even if where you're altering the theme registry (and need to see it reset) is from work you're doing on a module.
Remember to take it out again!
Installing devel module and using its reset cache function is probably easier and definitely better practice.