lighttpd unfortunately isnt a mature webserver yet. you'll lack the ability to read .htaccess files for example... users wont be able to configure applications on their own (keep in mind a lot of web software uses .htaccess for things like rewrites). there are a lot of other reasons why you probably should stick with apache in a multi-user environment like this, but i recommend you take a look through the lighttpd documentation to get a feeling for it all. if you manage the whole config thats one story, but cpanel is designed for allow the users to manage the details of their own accounts, etc, hence the control panel (and not just 1 big support request system where you actually change the configs)
you *might* be able to launch lighttpd on a seperate port *just* for the cgi of particular hand-configured hosts/url's using apache's proxy stuff to lighttpd. In converse you could force apache onto a funny port and simply have lighttpd proxy to the apache on the back end (running on a different port) but that sorta defeats the purpose
