Ray Booysen has just finished a big set of work where the Silverlight LocalMessaging classes where heavily used as messaging transport and he decided to share some information on the subject.
Without going into too many details as I'd break some contractual obligations, I can say that we needed a way for two Silverlight applications to communicate in a master/slave scenario. Typical messaging loads were in the region of around 1000 messages a minute with varying sizes with another requirement being guaranteed delivery and no out of order messages.