Settings needed to release your sbt-based Scala project against Scala 2.11.0-M1.

release-against-scala-2.11-M1.sbt

// release-against-scala-milestone v0.1

// ALTERED SETTINGS START

scalaVersion := "2.11.0-M1"

// disable cross building completely for releasing against milestones
crossScalaVersions := Seq.empty

scalaBinaryVersion in Global <<= scalaVersion.identity

resolvers += "Sonatype Release" at "https://oss.sonatype.org/content/repositories/releases"

// ALTERED SETTINGS END

// Cosmetics, info, etc. below
// some constatns are duplicated below because sbt 0.12.x doesn't allow to declare vals in .sbt files

onLoadMessage  <<= (scalaVersion, name, version) apply { (sv: String, name: String, version: String) =>
  val magnetText = "\033[35m"
  val defaultText = "\033[39m"
  val reset = "\033[0m"
  val boldOn = "\033[1m"
  val boldOff = "\033[22m"
  def colorize(s: String) = Predef.augmentString(s).lines.map(line => magnetText + line + defaultText).mkString("\n")
  // I wish I could use Scala's string interpolation below...
  colorize {
    """|
       |                    /
       |               ,.. / 
       |             ,'   '; 
       |  ,,.__    _,' /';  .
       | :','  ~~~~    '. '~        %1$sThe release-against-scala-milestone v0.1
       |:' (   )         ):,        %1$sis %2$senabled%3$s.
       |'. '. .=----=..-~ .;'
       | '  ;' ::    ':. '"  
       |   (:  ':     ;)     
       |    \\  '"   ./      
       |     '"      '"      
       |                (by DR J)
       |""".format(defaultText, boldOn, boldOff).stripMargin
  } +
    """|
       |
       |Your project %1$s-%2$s
       |is configured to be released against Scala-%3$s.
       |
       |Type %4$srelease-against-scala-milestone-info%5$s for more information.
       |
       |""".format(name, version, sv, boldOn, boldOff).stripMargin
}

commands += Command.command("release-against-scala-milestone-info") { state =>
  val msg =
    """|
       |Your build settings has been altered by `release-against-scala-2.11-M1.sbt` file.
       |Once you are done with releasing against Scala milestone simply remove that file.
       |
       |The following settings has been altered:
       |  crossScalaVersions, scalaBinaryVersion, resolvers
       |
       |In the future we'll fix sbt so it handles milestones correctly out of the box.
       |The file has been brought to you by Typesafe. If it doesn't work, feel free to ping
       |us at scala-internals mailing list.
       |
       |KNOWN ISSUES
       |
       |  1. Prints a warning about mismatching binary versions for Scala dependencies like
       |     library, actors or compiler. This is a bug in sbt 0.12.x that you can safely
       |     ignore.
       |""".stripMargin
  state.log.info(msg)
  state
}

Changing shellPrompt breaks idea-sbt-plugin which sort of 'screen scrapes' the output looking for the standard prompt. Maybe that's not such a big problem, just something to consider.

@retronym: thanks for letting me know. I probably will drop shellPrompt change as there's enough of info printed on other occasions.

I'm wondering why idea-sbt-plugin is doing it that why?