Configuring a extranet/intranet Sharepoint site behind ISA with Forms-Based Authentication and SSL

I am working on a very specific configuration for a Sharepoint site, so my question will also be very specific. Here's the requirement: I am trying to create a Sharepoint site with the front and several sub pages available publicly on the internet. Then, I need users to click Sign-In, and authenticate with their Active Directory ID's (which are already created). Upon login, they will have various levels of access to private team sites AND the ability to manage content of the private and public areas.

Here's the issue: the server sits behind an ISA firewall, which listens for traffic on port 80 and converts traffic to SSL (https). Internally, it communicates with the SharePoint server in regular http because there are no security threats within the network. I want users from the internet to hit the site on http://. Then, I want ISA to redirect the connection to https:// and display the public-facing SharePoint site with no login. This, I have gotten to work. ***However, when they click sign in, instead of Windows login dialog popping-up, I want them to be redirected to ISA's web-based form, and use their Active Directory logins to authenticate.*** I also tried to set up Sharepoint's web-based forms by modifying the web.config file, but this failed with an ambiguous error that I could not trace.

What is the best configuration to use in Sharepoint, IIS, and ISA to create this experience?

Thank you!

Versions:
Windows Server 2003 SP2
MOSS 2007
ISA 2006