The only real danger that I am aware of is if your script uses an uninitialized variable in some way such that if the user submits post or get data that would create that variable via register_globals, then possibly unwanted things could happen.
One way to check if this could be a problem is to turn on all error reporting including notices, and check to see if you get any notices about using uninitialized variables. If so, then that script should be modified to ensure that the variable in question is initialized (normally to an empty value) before it is used. This could be done globally at the php.ini level or at the directory level via .htaccess (if using Apache) by setting display_errors 1 and error_reporting E_ALL. Or you could do it at the script level with the following at the start of the script:
<?php
ini_set('display_errors', 1);
error_reporting(E_ALL);
Once done debugging, you should turn off display_errors so that things you might not want malicious users to see do not get displayed (and instead review your PHP error logs to see if things are running OK).
However, for forward compatibility, the better solution is to remove all dependencies upon register_globals and then turn it off, since it won't even be an option in PHP6.
