I have been having a lot of problems recently getting clean dist-upgrades from the repositories that have hosted the AMD64 port during it's development. I recently read a notice that these repos would be closed down shortly as the AMD64 release was incorporated into the official Debian repositories. The announcement said that Sid users should change their sources.list now, and that Etch users should do it 'soon'.
I decided to go ahead and upgrade to Sid since I was having a lot of problems with my mostly Etch installation. I changed sources.list to:
... like any 'official' Debian user, ran apt-get update, apt-get dist-upgrade ... and got a massive file replacement, after which all the problems I was having disappeared.
I will now set sources.list back to testing, assuming that it will be ready for any necessary upgrades when I am.