A read only bios seems kind of far fetched to me. M$ obviously made great efforts to shrink this board down. If the bios was read only (and therefore pre-flashed before being integrated) the LPC would be gone. That's the only purpose the LPC serves to M$. So, wherever the new flash rom is (inside the Xyclops or on the underside), it is still going to be flashable, and it will still be flashable from the LPC bus. The factory Bios has to get there somehow, people.
Besides, why would M$ order X million units of a read-only bios, when if they updated the firmware, they'd have to order a whole new batch of chips? They are trying to cut costs, not increase waste.
The Bios being on the HDD is a total impossibility. The Bios controls HDD functions (not to mention unlocking the HDD so it can be accessed), so it's not possible for the XBox to load a Bios from the HD.
The serial EEPROM that everyone keeps thinking could be large enough for a Bios could not be. It's only 256kbits, maybe 512kbits. The old TSOP on a 1.2-1.4 was 2Mbits, or 256kbytes. 1 byte = 8bits. Besides, that same EEPROM was on all the previous XBox's. I'm not certain, but it probably holds the key for your locked HDD.
Underboard shots will be necessary. Hands down. Without those, though, these are about as solid a fact you can get from the previous pictures, and the way that computers function.