The download page at scala-ide.org for the pre-configured Scala IDE for Eclipse (4.0.0 Release) states that its content includes SBT 0.13.6. I downloaded the Mac OS X 64-bit version, and found that the /plugins directory indeed contains a jar file called org.scala-ide.sbt.full.library_0.13.6.v-2... Is this a replacement for having a separate installation of SBT on one's development machine, perhaps facilitating some tighter integration between the Scala Eclipse plugin and SBT? If so, how does it work?
Yes, it does indeed include sbt. It is used internally to control the build, notably the incremental compiler.
It isn't a replacement for having sbt installed on your machine, it isn't really accessible from outside scala-ide.
As for tighter integration, not at the minute.