http://debian.slimdevices.com/dists/
http://http.us.debian.org/debian/dists/
Why not choosing another mirror that can handle codenames? Or is that a
special mirror, different (non synced) from Debian ones?
For standard mirrors, should you want to keep Lenny (and as soon as
Squeeze comes out), if you are still pointing your repos to "stable"
you'll get into troubles.