Hi, I'm curious about mit-scheme on amd64's progress as well.
I've been able to install mit-scheme on amd64 by dowloading the i386 package and installing with dpkg --force-architecture -i, though this is not ideal. Knowing nothing about package maintenance, and armed with the very basic TLDP howto ( http://tldp.org/HOWTO/html_single/Debian-Binary-Package-Building-HOWTO/) and not enough time, I managed to build mit-scheme on my amd64 system and make a package that passes a lintian check. You can download it here http://www.gmclean.org/mit-scheme_9.0.1-1_amd64.deb. Let me know if it works, if I'm a complete putz, etc. Hope it helps some folks, works on my amd64 machine. Best wishes, -Garrett