dnottis:
Did you clear the CMOS after flashing?

I had tons of problems if I tried just starting the board after a flash, and using settings without a cmos clear first.

About the B2 vs B3 swap:
Seems to be all over the map. No problems at all on my UD5. Saw some Asus people say that the stability was exactly the same, one who said he was able to LOWER the vcore by 0.015v on overclock.net, and many saying less stable. But I don't think anyone who said it was less stable, even tried measuring voltages with a DMM. You'd need readings from both boards to make it mean anything.

But I saw these same complaints when people were just flashing different bioses on the B2 boards. The hardware is identical besides the PCH, right?

What someone really needs to do is take a DMM to the board and smack the vcore readings and see if the boards are really outputting the same voltages...