In the same press release as the Expression Web announcement, Microsoft also announced a new CTP for the Windows Presentation Foundation/Everywhere, codenamed 'WPF/E', overlapping the WPF codenamed 'Avalon'.
This interview between Expression PM Arturo Toldeo and Brazilian Interactive media designer Celso Gomez, gives a breakdown on what WPF/E really is and how it relates to XAML, AJAX and the future.
There is a separate CTP for Windows and Mac, as well as a Sample Pack and SDK:
“WPF/E” (codename) Community Technology Preview for Windows (Dec 2006)
“WPF/E” (codename) Community Technology Preview for Mac (Dec 2006)
“WPF/E” (codename) Software Development Kit (SDK) Community Technology Preview (Dec 2006)
“WPF/E” (codename) Community Technology Preview Sample Pack (Dec 2006)