This is something I do at least two or three times a day:
Ctrl-C should be captured and defined to cancel the currently-running task, or exit if no task is running.
IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#228 for the full issue and to comment.