Sorry, should've been more clear.
I've tried different versions of pfSense only to find one that would work, which was a 1.2.3 RC1 release but somehow the filesystem layout for this one was so messed up that I couldn't do an upgrade. All of the other versions either wouldn't find the boot loader (No /boot/loader found) or would boot but would end up missing the root filesystem.
I've eventually tracked down the problem I think:
If you're trying to write the image file to a CF disk from an OS X machine, OS X will try at random intervals to mount that image making the disk unavailable until you click "Ignore". Because of this, the whole writing to disk fails but you don't see that from the output. (I've tried using Disk Utility to burn the image to the CF card but that one borks as well saying the image is corrupt)
I've eventually been able to write a 2GB image (1.2.3-RELEASE) to a 4GB flash card from a spare Linux laptop I have lying around and that worked fine so the end result is that OS X keeps trying to mount a device (or at least do something with it) that messes up the writing of the disk image.
Not sure yet how to be able to write an image from OS X without it trying to mount it continually but I'll figure that one out sooner or later.
At least, now I've got a working pfSense machine.
Sorry about the confusion but this was really strange.
Regards,
Jeroen