On my OpenBSD desktop, I configured apmd to log every minute at /var/log. I also configured sensorsd.conf to set a high temperature value that, when passed, a message should be logged.
Then, I run an awk script at X login that reads apmd log (from tail -f) and writes a notification to xnotify's fifo when a interesting line is read.
It currently only greps for lines reporting low battery and high temperature (see the image below, at top right).

For my dock/bar I use WindowMaker-style dockapps, in particular wmbubble, which compiles some CPU and memory info as a duck swimming on a pool of bubbling water. When a mouse is over it, wmbubble shows the load average graph.