EDIT: some guy (Chris) proposed a genius idea:
Get a paperclip and carefully insert into the VGA header plug, shorting pin 6 to pin 12 (middle row far right to bottom row 2nd from the right). Whenever you don't have a monitor attached leave the paperclip in. Your Alix 3c3 will now boot up without a monitor attached.
So greats to him :).
According to Wikipedia, Pin 6 = Red return and Pin 12 = PC Data, then it makes sense to bridge them.
Apparently, CONFIG_FIRMWARE_EDID=n should fix it, too.