This works if you change line 10 (var dbVar = database) to val dbVar = database

build.sbt

// Set the project name to the string 'My Project'
name := "SBTProject"
 
// The := method used in Name and Version is one of two fundamental methods.
// The other method is <<=
// All other initialization methods are implemented in terms of these.
version := "1.0"

scalaVersion := "2.10.1"

resolvers += "Sonatype Snapshots" at "http://oss.sonatype.org/content/repositories/snapshots/"

resolvers += "Typesafe Stack" at "http://repo.typesafe.com/typesafe/releases/"

libraryDependencies ++= Seq(
  "org.reactivemongo" %% "reactivemongo" % "0.9-SNAPSHOT",
  "play" %% "play-iteratees" % "2.1.0"
)

sbt-output

> run
[info] Compiling 1 Scala source to /home/joern/Desktop/reactivemongo/target/scala-2.10/classes...
[error] /home/joern/Desktop/reactivemongo/test.scala:11: type mismatch;
[error]  found   : reactivemongo.api.collections.default.BSONCollectionProducer.type
[error]  required: reactivemongo.api.CollectionProducer[C]
[error] Error occurred in an application involving default arguments.
[error]     val collection = dbVar("somecollection")
[error]                           ^
[error] one error found
[error] (compile:compile) Compilation failed
[error] Total time: 4 s, completed Apr 4, 2013 2:42:40 PM

sometest.scala

object Main {
  def main(args: Array[String]) {
    import reactivemongo.api.MongoDriver
    import scala.concurrent.ExecutionContext.Implicits.global

    val driver = new MongoDriver()
    val connection = driver.connection(List("localhost:27017"))
    val database = connection("test")

    var dbVar = database
    val collection = dbVar("somecollection")
  }
}