It's needed, but it's not obvious from the Castle homepage where this file sits. I am looking for a binary release, not source code.
HornGet is a good option:
http://hornget.net/packages/ioc/castle.facilities.activerecordintegration/castle.facilities.activerecordintegration-trunk
Castle's TeamCity build server is another option for getting binaries:
http://builds.castleproject.org/
http://builds.castleproject.org/repository/download/bt87/5399:id/Castle.Facilities.ActiveRecordIntegration-.NETFramework-v3.5-49.zip