This has been spinning in the back of my mind for a couple weeks now, but seeing as how Twitter uses a message queue internally and is available for free, it seems that you could use it to have disparate (separately homed) applications communicate to each other using Twitter accounts to send messages. The messages are preserved in order, although limited to 140 characters and not secured.
So I checked around and I see that I’m not the only person who has considered this. Todd Clayton’s post even goes into some detail about how to build such a service, how to support larger messages (by splitting or with external tools like TwitPic).
Mostly, though, it’s a Gedankenexperiment. Clearly it’s just a solution looking for a problem.