My home automation system is all based around XMPP, with a standard Jabber server to which all the home automation systems connect to share messages. I wrote it like this so that it would be easy to just take some existing Jabber client for a platform and be able to come up with a nice looking front end with minimal effort.
I found Iksemel, a portable C XML parser and protocol library that looked perfect, and it only took a couple of hours to have it ported on the NDS, and a couple more hours to get it working with PAlib for wifi. It's not a generic Jabber chat client, but it wouldn't take too much work to make it into one (although I didn't bother with encryption support so you won't be able to use it with Google talk servers for example). Anyway, the code might save someone a few hours, so I've made the source available.
Anyway the idea was to look for something that would use standard components, where frameworks existed in Perl and C for me to write simple code, and to work on the principle of messaging - the UPS for example would respond to status requests and give you things like the temperature and voltage; with a heartbeat notification with the status included every minute; but with urgent alarms to anyone who registers an interest in getting them. Whats the solution? Jabber! In about an hour I had a jabber server running and a test Perl client doing just that; this thing will rock :)