pfSense 2.x runs on FreeBSD 8, instead of the previous 7. I wanted to get istatd working on my router, so I fetch'd libxml2 from here and the istatd binary (version 0.5.7) compiled against FreeBSD 7 from my previous blog post, here. For this to work, you'll also need the iOS app. When running istatd, it complained about some libraries. I simply created symlinks from the old library to the new one and everything worked fine, see below.
ln -s /lib/libz.so.5 /lib/libz.so.4
ln -s /lib/libkvm.so.5 /lib/libkvm.so.4
You can get a template for the configuration file here and put it in a nice place, like /etc/istat.conf
Create your user, group and socket file and you're good to go:
pw user add -n istat
pw useradd istat -G istat
Then, you can run the istatd daemon with just
istatd -d -c /etc/istat.conf
Now, you'll be able to monitor your pfSense box with iStat!