This may be due to a change that stumped me a bit too today when I was upgrading to 2006 and wanted to install OpenSSH. Try changing the info for http://repository.maemo.org from "mistral-beta" to "mistral" and then try and see if it works.

404 of course means file not found, ie you have tried to access some resource that isn't there - and of course, the 2006 OS is out of beta so removing the "beta" part makes sense.