The problem was including the scalaVersion setting inside the plugins.sbt file. This causes sbt to search for sbt-plugin_2.10.0_0.12 in the repositories when it should actually search for sbt-plugin_2.9.2_0.12.
I'm not sure about the semantics behind specifying the scalaVersion in the plugins.sbt file, but maybe it's declaring the Scala version that SBT is running on.
Here is a link to the Play 2.1 sbt-plugin files: http://repo.typesafe.com/typesafe/simple/ivy-releases/play/sbt-plugin/scala_2.9.2/sbt_0.12/2.1-RC1/srcs/