So I installed Boot Camp when it was still in the beta versions on an OS 10.4 MacBook about 2 years ago but never install a Windows partition. Recently I tried to install Windows, so I started Boot Camp Assistant only to be greeted with a message that it had expired. As with most Apple stuff, there isn’t much support if you don’t upgrade (and in the process pay Apple more money). After some searching I found this forum post which basically states to change the date to 2005 (System Preferences > Date & Time) and ensure “Set date & time automatically” is unchecked.
I can verify that it works with OS X 10.4 on 20th August 2010 (Note: Boot Camp Assistant was previously installed so I just needed to run it, not install it. I can’t seem to find any download links to old Boot Camp versions either)