When I turn off allow_url_fopen in php.ini the safe_mode UID check seems to be disabled. 

With "allow_url_fopen = on" an include("/etc/passwd") returns the following error:

"The script whose uid is 10000 is not allowed to access /etc/passwd owned by uid 0"

after I've changed the settings to "allow_url_fopen = off" the inclusion works fine, so there is no way to prevent customers from including external files and local system files.