Hi, I also think this would be nice to have. Adding an extra field to sources.list is simpler than having to maintain an additional file for the pinning.
Besides the common use case of adding unstable to a system running Debain testing, it would also make it easier to add additional suites to a system running stable to pull in debian-installer-*-netboot-* packages for multiple releases. It would also be nice if there were named values as well, like Pin-Priority: not-automatic Pin-Priority: automatic-upgrades having similar effect as "NotAutomatic: yes" respectively "ButAutomaticUpgrades: yes" in Release. (Obivously choosing a nice name is hard.) Ansgar
