I have a m0n0wall running 1.23b1 right now.  I went through the standard 
upgrade on the webgui to 1.3b1 and it would no longer boot.  Upon 
looking at the boot screen, I see the word "boot" with a random amount 
of arrows to the right of it each time I tried.  However, it would 
always freeze at that point.  I took the CF card out, and wrote it 
manually with physdiskwrite.  Still it would freeze at the same point.  
Used physdiskwrite to put 1.23b1 back on, all is well.  The card I am 
using is a Toshiba 128MB card.  The machine has 256MB of ram in it and 
is a Celeron 566 chip.

Chris