Florin just emailed me requesting to help on letting the word out on with the solution for this problem he was facing, so here it is, on his own words:

«If you upgrade from WSS 2.0 to Wss 3.0 and configure WSS 3.0 with anonymous access, you might get the following error while trying to create websites:

Error: File not found.

 

Looking at the Verbose Logfiles, you might get the following Error message:

Application error when access /_layouts/newsbweb.aspx, Error=A aplicação Web não foi encontrada em http://www.myportal.com/blogs/2a.

Verifique se escreveu o URL correctamente.

Se o URL tiver de fornecer conteúdo existente, o administrador do sistema poderá ter de adicionar um novo mapeamento do URL do pedido à aplicação de destino.

em Microsoft.SharePoint.SPSite..ctor(SPFarm farm, Uri requestUri, Boolean contextSite, SPUserToken userToken)

em Microsoft.SharePoint.Administration.SPNativeConfigurationProvider.LookupSite(String requestUrl, Int32 iisInstanceId, String& siteUrl, Int32& zone, Guid& appId, Guid& siteId, Guid& databaseId, String& databaseServer, String& databaseName, String& databaseUsername, String& databasePassword, Boolean& useHostHeaderForSiteName) ........

 

Resolution:

 

1. ISA 2004 Web Publishing Rule (additional configurations over  the default rule):

    A. Forward the original host header instead of the actual one

    B. Requests appear to come from the original client

 

2. WSS 3.0 Configuration:

    In SharePoint 3.0 Administration page -> Operations -> Go to Managed Paths (Mapeamentos de Acesso Alternativos)

    Set the Internet Url equal to the public FQDN of the website used by ISA 2004 to publish the your portal, e.g. www.myportal.com.

 

3. IIS Configuration:

    Add the public FQDN as Host Headers in IIS for the website extended with WSS 3.0, e.g. www.myportal.com.

 

I hope that this can help someone.

 

Greetings!
Florin Zamfir»

Cool, eh?