Running sensors-detect produces the error: 
 
No i2c device files found. Use prog/mkdev/mkdev.sh to create them. 
 
I debugged the script and noticed that it was not setting $use_devfs 
even though it is my understanding that FC3 uses it.  It seems to be 
scanning /proc/mounts for the string "devfs" but mine does not 
contain it, as follows: 
 
rootfs / rootfs rw 0 0 
/proc /proc proc rw,nodiratime 0 0 
none /dev tmpfs rw 0 0 
/dev/root / ext3 rw 0 0 
none /dev tmpfs rw 0 0 
none /selinux selinuxfs rw 0 0 
/proc /proc proc rw,nodiratime 0 0 
/proc/bus/usb /proc/bus/usb usbfs rw 0 0 
/sys /sys sysfs rw 0 0 
none /dev/pts devpts rw 0 0 
none /dev/shm tmpfs rw 0 0 
/dev/hda3 /home ext3 rw 0 0 
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0 
sunrpc /var/lib/nfs/rpc_pipefs rpc_pipefs rw 0 0 
nfsd /proc/fs/nfsd nfsd rw 0 0 
 
I modifed the script by adding inserting $use_devfs = 1; at line 
1786, and then it worked. 
 
Side comment: this is my first time using lm_sensors, but I was 
surprised how difficult it was to set up.  I guess by the old time 
do-it-yourself Linux standards it's easy (run a few scripts, press 
enter a lot at a bunch of questions I don't really understand, 
cut-n-paste some text into a file) but by the new, 
everything-just-works standard that Fedora and other modern distros 
use it seems like a lot of busywork.  Shouldn't sensors-detect run at 
install time, answering all the questions with their defaults 
automatically, and setting the appropriate modules to load?  Or at 
the very least, have an /etc/init.d/lm_sensors script which has an 
"init" option that does all of this.