Is your internet connection fine? (Also if you are trying to modify the archive beforehand, 7Zip corrupts it)

The way I do it (and I was doing this for 3 hours in the early morning, trying to modify the bootmenu.sh)
is to save the http://fanoush.wz.cz/maemo/initfs_flasher.tgz on the internal memory card using my computer, then:

cp /media/mmc2/initfs_flasher.tgz /home/user/MyDocs/.documents
cd /home/user/MyDocs/.documents
tar zxvf initfs_flasher.tgz
cd initfs_flasher/
./initfs_flash

The archive has to be extracted on the built in flash memory of the N800 and also I cant just zip up the files for you because the initfs flasher relies on symlinks etc which tar recreates.