just want to share my solution for this issue.
it tooked me about 3 hours but
finally i got pfsense installed on my new itx machine.
Here is what i did:
Step 1: Download Multiboot USB ISOhttp://www.pendrivelinux.com/boot-iso-from-usb-flash-drive/
and use it
Step 2: copy the pfSense.iso onto the stick
Step 3: update the menu.lst as follows...
title pfSense 1.2.3
find --set-root /pfSense.iso
map /pfSense.iso (hd32)
Step 4: extract the pfSense.iso to the stick (this is needed because Multiboot emulates an cdrom which isnt present when the boot routine checks the hardware)
Step 5: boot from the stick. if grub does not show up the list with the iso just type >find --set-root /menu.lst press escape to return to the list and press enter
you also can type the following commands in the grub console for booting the iso manualy
>find --set-root /pfSense.iso
>map /pfSense.iso (hd32)
now pfSense should boot
Step 6: as described in step 4 the boot loader cant take notice of the emulated cdrom so you have to mount your usb stick manualy
just type msdosfs:da2 (or whatever your devicename is. you can check by typing ?)
finally the install routine schould start
i hope this will help somebody to save some time!