Actually I got it.
if you have a project with build.sbt (that uses 2.10.1 scala) file - as soon as you type sbt.. all dependencies will be downloaded into ~/.sbt folder - even scala compiler will be downloaded there (~/.sbt/boot). It could be even several version of scala: 2.10.1 and 2.9.2 for example.
and about sbt-idea and ~/sbt/plugins .. it could any scala version - depending on its build.sbt file, for example in my case:
resolvers += "Sonatype snapshots" at
"http://oss.sonatype.org/content/repositories/snapshots/"
addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.2.0-SNAPSHOT")
I should notice if try different version.. like 1.1.0-M2-TYPESAFE it will not work.. (at least in my case) - gen-idea command is not available then. I do not know why. I guess it should.
Also if you do not point resolvers += - it will will not work.. but it it will not tell you about that..
This plugin is using scala 2.9.2 - we can not see it here, but we can see it from that outputs it produces while installing/downloading. That's why we have ~/.sbt/boot/scala-2.9.2/ as a result.
In any case we should not care about it. It is handled by sbt.
When you converted your sbt-project into your intellij-idea project by typing gen-idea in sbt console, as the result your IDE project will be referencing to ~/.sbt/scala but not to your somewhere-installed-scala.. So even no need to pointing the scala location - that sbt-idea sbt's plugin will do all the work. And that's good!
That's the answer I wanted to get. One gets/understands it by trying it.
