I just discovered (via Sam Ruby) Rohit Khare's work on application layer internetworking, or ALIN. Rohit gave a talk at the O'Reilly Emerging Technologies conference last May and has a powerpoint presentation and some rough notes online. What I've been calling Layer-5 routing, Rohit calls Layer-7 or ALIN. I'll defer to him since he's got the powerpoint done (not to mention that he's probably thought it through more).
Reading Rohit's presentation, I realize I left out a very important feature in my thoughts on ALIN: message store and forward. I was thinking of transport as orthoganal to the idea of ALIN. I still think that maybe it is (at least for the feature set I envision), but Rohit argues that transport can't be ignored because latency is such a huge issue in web services. He's got a point.
Rohit calls out the following companies as being in the "Internet-scale" messaging business: KnowNow (which Rohit founded), Bang Networks, Kenamea, SonicXQ, and Grand Central. If you visit these sites and look at their customer lists, you'll notice that they all sell to a lot of financial institutions and brokerages. This doesn't surprise me. When we were building credit card gateways for First Data Corp. at Excite\\@Home, we were using IBM MQ Series to route transaction records among geographically dispersed gateways. Financial institutions have been chasing this problem for years.