With current stable xorg-server-1.15.0, setting 'DISPLAYMANAGER' to 'sddm' results in sddm running in the foreground. sddm will log all output to vt1 and it will make other vt's inaccessible. Although sddm "works" fine with current xorg-server, I believe sddm should depend on >=x11-base/xorg-server-1.15.1 because xdm init script support for sddm was added to xorg-server-1.15.1. See bug #498972.
Please post your `emerge --info x11-misc/sddm' output in a comment.
Thanks for reporting, fixed.