"SQL Server 2005 comes with a new service that is very useful for creating robust Enterprise applications: the Service Broker. Service Broker is useful to implement guaranteed and in-order delivery of messages between two services. This is modeled as a dialog between two services. Currently, Service Broker lacks the support for monolog, or simple queuing of messages."

http://www.codeproject.com/sqlrs/SimulatingMonologConverse.asp