I don’t know why, but everything associated with Ruby on Rails seems to change all the time, without notice, or (in the case of URLs) without redirect.
We used http://svn.rubyonrails.org/rails for our svn:external deployment of rails 1.2.6. But that suddenly stopped working. The Ruby on Rails weblog doesn’t contain any information on it (that I can find), going to svn.rubyonrails.org with my web browser yielded just the front page of the weblog and Googling didn’t help much. That is, until I accidentally found http://dev.rubyonrails.org/svn/rails.
So, I changed the svn:externals from “rails http://svn.rubyonrails.org/rails/tags/rel_1-2-6/” to “rails http://dev.rubyonrails.org/svn/rails/tags/rel_1-2-6/” and now it works again.