I have been playing with logging this week. FHEM now automatically records the measured temperature, the desired temperature (set point) and actuator levels for each room to Pachube.com (pronounced Patch Bay). With the data held on Pachube, I can do various things like build dynamic graphs for display on this site. At the moment, I am using a Pachube app which displays the graphs using flash, but being an iOS user, this is less than optimal.
You can view the current temperatures in my three bed terrace by clicking the
following link: Household Temperature Graphs