At the RabbitMQ project, we have run into some interesting related issues. RabbitMQ implements AMQP and has adaptors for
other tech such as XMPP, SMTP, HTTP.

Your article made me wish to share with you some of our thinking on
this:

A) Comparing protocols

I like the assessment. Here is ours The second presentation
linked to here contains a detailed comparison of messaging protocols that you
may find useful:
http://google-ukdev.blogspot.com/2008/09/rabbitmq-tech-talk-at-google-lo
ndon.html

Here is a great post about how last.fm use IRC for a lot of
integration infrastructure:
http://www.metabrew.com/article/how-we-use-irc-at-lastfm/

B) “challenges with XMPP”

We worked out how to combine XMPP (IM, federation, potentially MUC)
with AMQP to provide ‘drop in pubsub’ here:

* (integration)
http://hg.rabbitmq.com/rabbitmq-xmpp/raw-file/default/doc/index.html
* (comment)
http://anders.conbere.org/journal/rabbiter-open-federated-pubsub-server/

Also – have you looked at Nanite and Vertebra from Engineyard? These
are relevant to your work. Various links are easily found on Google, but you can start with Ezra’s blog at http://brainspl.at

Cheers,

alexis