When you set safe mode, people are able to include files if:
(1) The ownership of the file matches the ownership of the php script being executed. Ditto for readfile(), include_once(), etc.
(2) The included file is in a path set in safe_mode_include_dir (the configuration setting).
Now, open_basedir is actually a directory where, if set, restricts all file operations, such as fopen() and the like. I'm not sure whether including something in open_basedir will permit an include(), overriding the uid/gid matching, but you can get around the whole issue by setting a safe_mode_include_dir (and it probably wouldn't hurt to make sure it included "." as part of it, so files in the same directory as the script can be included).