repl.scala

scala> util.Properties.versionMsg
res0: String = Scala library version 2.9.0.final -- Copyright 2002-2011, LAMP/EPFL

scala> :load /Users/teamon/Desktop/test.scala
Loading /Users/teamon/Desktop/test.scala...
import java.io._
defined module Levels
defined class Entries
encode: (msg: Any)Array[Byte]
decode: (bytes: Array[Byte])Any
test: (a: Any)Unit
Entries(Map(a -> Trace))
[B@1235047f
Entries(Map(a -> Trace))

Entries(Map(a -> Trace, b -> Trace))
[B@507895d8
Entries(Map(a -> Trace, b -> Trace))

Entries(Map(e -> Trace, a -> Trace, b -> Debug, c -> Trace, d -> Trace))
[B@77b993d6
Entries(Map(e -> Trace, a -> Trace, b -> Debug, c -> Trace, d -> Trace))


scala> 

sbt-console.scala

scala> util.Properties.versionMsg
res0: String = Scala library version 2.9.0.final -- Copyright 2002-2011, LAMP/EPFL

scala> :load /Users/teamon/Desktop/test.scala
Loading /Users/teamon/Desktop/test.scala...
import java.io._
defined module Levels
defined class Entries
encode: (msg: Any)Array[Byte]
decode: (bytes: Array[Byte])Any
test: (a: Any)Unit
Entries(Map(a -> Trace))
[B@56f4defc
Entries(Map(a -> Trace))

Entries(Map(a -> Trace, b -> Trace))
[B@60051f44
Entries(Map(a -> Trace, b -> Trace))

Entries(Map(e -> Trace, a -> Trace, b -> Debug, c -> Trace, d -> Trace))
[B@5f297ee0
java.lang.ClassCastException: cannot assign instance of scala.collection.immutable.HashMap$SerializationProxy to field Entries.entries of type scala.collection.immutable.Map in instance of Entries
	at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2039)
	at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:1212)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1952)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
	at .decode(<console>:12)
	at .test(<console>:15)
	at .<init>(<console>:17)
	at .<clinit>(<console>)
	at .<init>(<console>:11)
	at .<clinit>(<console>)
	at $export(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:592)
	at scala.tools.nsc.interpreter.IMain$Request$$anonfun$10.apply(IMain.scala:828)
	at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
	at scala.tools.nsc.io.package$$anon$2.run(package.scala:31)
	at java.lang.Thread.run(Thread.java:680)

scala> 

test.scala

import java.io._

object Levels extends Enumeration {
    val Trace, Debug, Info, Warn, Error = Value
}

case class Entries(entries: Map[String, Levels.Value])

// object Test {
    // def main(args: Array[String]): Unit = {
        def encode(msg: Any) = {
            val bos = new ByteArrayOutputStream
            val out = new ObjectOutputStream(bos)
            out.writeObject(msg)
            out.close

            bos.toByteArray()
        }

        def decode(bytes: Array[Byte]) = {
            val in = new ObjectInputStream(new ByteArrayInputStream(bytes))
            val obj: Any = in.readObject
            in.close
            obj
        }


        def test(a: Any){
            println(a)
            println(encode(a))
            println(decode(encode(a)))
            println
        }


        test(Entries(Map("a" -> Levels.Trace)))
        test(Entries(Map("a" -> Levels.Trace, "b" -> Levels.Trace)))
        test(Entries(Map("a" -> Levels.Trace, "b" -> Levels.Debug, "c" -> Levels.Trace,
                                "d" -> Levels.Trace, "e" -> Levels.Trace)))
    // }

// }

Tested in both sbt 0.7.7 and 0.10.1, and scala 2.9.0 & 2.9.0-1