From what I understand SID installs are only "unofficial". I tried on 2 different occassions to install sid directly, but it failed miserably.
No part of the installer exept the bits about handling modules and stuff worked. I had to set up networking by hand, had to partition/format/mount all the partitions completely by hand. Eventually I just gave up on the installer completely and used the debian starter tarball thingi and installed it Gentoo-style ( untar base system to mount; chroot into partition; set up apt/bootloader/networking, then used dselect to install and upgrade everything I needed.)
Generally what Debian wants you to do is to install the current stable (just a minimal install, no X or anything yet) and then upgrade it to SID by changing the /etc/apt/source.lst and doing a apt-get update/apt-get dist-upgrade. Probably helps to keep down the unknown variables to try to get people to install that way. After all it is "unstable". I installed straight from unstable, though, and it worked out just fine.