I want to try the "viewer-less" viewing of DWFX files in Vista but I have found no way of publishing DWFX files from AutoCAD. There seems to be only DWF, without X. Is it only renaming of the file or is the format different?

Yes, the DWFX format is different (XML-based). There is a free add-on DWFX driver for AutoCAD 2008 (both for the 32-bit and 64-bit version) - see:
http://www.cadforum.cz/cadforum_en/download.asp?file=DWFX