You can also call the shell and manually call the PHP file. No cron required and no waiting.
http://www.php.net/manual/en/function.exec.php
From the Notes
Section
“If a program is started with this
function, in order for it to continue
running in the background, the output
of the program must be redirected to a
file or another output stream. Failing
to do so will cause PHP to hang until
the execution of the program ends.”