sbt 0.7.7 is released[1]. The only change from 0.7.5 is compatibility with Scala 2.9.0's improved REPL[2]. Please obtain a new version of the launcher here:
http://simple-build-tool.googlecode.com/files/sbt-launch-0.7.7.jar
and set your sbt.version in project/build.properties to 0.7.7. If you have already used Scala 2.9.0 on an existing project with an older version of sbt (such as 0.7.5), you will need to clean project/boot/ first.
Thanks,
Mark
[1] This release is what was supposed to be 0.7.6 but had to be bumped to 0.7.7 due to deployment issues.
[2] http://code.google.com/p/simple-build-tool/issues/detail?id=156