Apache2, php5-fcgi, php4-fcgi, mod_fastcgi HowTo provides everything you need to know to run PHP as an FCGI process. And even in German. The little bit of Apache2 in there can be mentally converted to Apache 1.3, the Apache is actually hardly affected.
FCGI offers, in combination with suexec, the possibility to run PHP per virtual host under a dedicated user and thus the possibility in shared hosting environments to set up files in a virtual host so that another user with his PHP cannot read them. You could even run the FCGI-PHPs in a chroot jail to isolate them even more.
In addition, FCGI is often significantly more resource-efficient for PHP, as fewer PHP processes can run than Apache processes and the Apache processes do not become so bloated. If you have many virtual hosts, this can lead to the FCGI processes catching up in number - but then you should consider whether the FCGI processes should not run better on a dedicated machine.
This would be exactly the right thing for simon, especially since I could then also allow PHP for the other users.