Hi now the file can can be found at http://www.swer.it/~attilio.fiandrotti/gtkx/d-i_xsrv.tar.gz(the name has changed since i didn't realize the tar on the netinst cd could not untar bzip archives)
i've configured the X server to use by default the vesa driver; anyway chip-specific drivers are still included in the package, so if you experience problems you can use them just modifiyng the X configuration file.
ciao Attilio