I've been trying to decipher exactly what fuse settings are made in the bootloaders. I found the
following in "Makefile" in the IDE optiboot directory,
atmega328_isp: atmega328
atmega328_isp: TARGET = atmega328
atmega328_isp: MCU_TARGET = atmega328p512 byte boot, SPIEN
atmega328_isp: HFUSE = DE
Low power xtal (16MHz) 16KCK/14CK+65ms
atmega328_isp: LFUSE = FF
2.7V brownout
atmega328_isp: EFUSE = 05
atmega328_isp: isp
This would explain why I can run the chips at 3.3V without the brownout triggering.
I've not yet found whether the watchdog is enabled or not. Takes a lot of searching to nail this stuff
down.