Stepping 5 ES chips require 0502 BIOS or higher. I don't know about retail chips. My CPU would not boot with 401 BIOS. But I already had a Cedar Mill chip, so I used it to boot up and update BIOS.
I think a few people with earlier steppings were able to boot up with 0401. You can always ask Asus to send you a new BIOS chip that supports Conroe.
There should be a yellow sticker on the BIOS chip. It will indicate what BIOS the board was shipped with.