According to Microsoft there are no FrontPage 2003 server extensions as such and there will not be any. Development efforts will be focussed on SharePoint Services, which include the server extension functionality. FrontPage 2002 server extensions will continue to be supported but not improved.
From their FAQ at
http://www.microsoft.com/office/frontpage/prodinfo/faq.mspxQ. I've heard that there are no FrontPage 2003 Server Extensions. Is that true?
A. Microsoft has continued to make investments in FrontPage 2003 to enhance the overall customer experience, based on customer feedback and the changing role of server extensions in our overall product offerings. As we examined our options, it became clear that the best solution was to create a new and radically more powerful server story—Windows SharePoint™ Services—and eliminate the need for server extensions beginning with FrontPage 2003.
Today, the optimal combination for FrontPage is to use FrontPage 2003 in combination with Windows SharePoint Services. This allows you to access the entire set of new functionality in FrontPage 2003, along with improvement to the way you did things in FrontPage 2002 and other previous versions of FrontPage. For more information about these changes, see FrontPage server configuration options and features.
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This chart compares the features available with the different levels of server extensions.
http://www.microsoft.com/office/frontpage/prodinfo/servers.mspx