Having seen implementations of MSMQ, SMTP and UDP transport providers for WSE, I was sure someone would come up with one for Service Broker as well. I played around with this for a bit but only got to implement the sender side. Mike Taulty recently released a Service Broker sample transport provider. I haven't tried it myself yet but I will post my comments on this soon.
Going forward, it will be possible to overlay Indigo on top of Service Broker in a similar fashion, using the Service Broker technology as a full duplex, transacted, secure and reliable channel. Anyone up for building a Service Broker Channel for the Indigo CTP bits?