Emma Healy of Emma Explains Microsoft Licensing in Detail has written a post on how to calculate what Microsoft Office SharePoint Server 2007 license you should use for your public facing/extranet MOSS site.
The post has a flow chart to aid you in your decision to choose a MOSS for Internet Sites license (expensive!) or CALS (less expensive). To sum it up you should use the MOSS FIS if you have more than 435 users when using MOSS Standard or more than 242 users when using MOSS Enterprise.
Note; if you are making a web site with MOSS for an unknown amount of (anonymous) users, your only way is to go for the MOSS FIS license.
I only whish that there was some license in between for small businesses or a version of MOSS with only the publishing features.