Updated 16 nov 2012
I wanted to have low voltage (brown-out) detection on my Arduino Uno R3 to prevent it from unexpected behaviour or even destroying my sketch when the 5V power supply drops below a certain level. I found it difficult to find the information because it was scattered around several arduino forum threats, therefore I created this page. This page is to show how to correctly set the brown out detection. To set the brown-out detection a new bootloader with the setting must be burned to the Uno. I have used 1 Arduino Uno to burn a bootloader to another Arduino Uno. The other Uno is used as an in-system programmer(ISP). Here's a description how to do that http://arduino.cc/en/Tutorial/ArduinoISP.
In the Arduino IDE (mine is 1.0.1) boards.txt contains bootloader settings of many different Arduino boards. On my computer the file is located here D:\programs\arduino-1.0.1\hardware\arduino\boards.txt. In that file you need to find the Uno.
##############################################################
uno.name=Arduino Uno
uno.upload.protocol=arduino
uno.upload.maximum_size=32256
uno.upload.speed=115200
uno.bootloader.low_fuses=0xff
uno.bootloader.high_fuses=0xde
uno.bootloader.extended_fuses=0x05
uno.bootloader.path=optiboot
uno.bootloader.file=optiboot_atmega328.hex
uno.bootloader.unlock_bits=0x3F
uno.bootloader.lock_bits=0x0F
uno.build.mcu=atmega328p
uno.build.f_cpu=16000000L
uno.build.core=arduino
uno.build.variant=standard
##############################################################
A change must be made to one of these lines to set the fuses of the Uno.
To find the fuse settings this fuse calculator is useful. http://www.engbedded.com/fusecalc. For Uno select ATmega328P. The second table shows that the brown-out detection (BOD) level is set with the extended fuse. Bits 0 to 2 are used to set the voltage level. Bits 3 to 7 are not used. Playing around with the calculator shows that 0x04 is 4.3V, 0x05 is 2.7V, 0x06 is 1.8V and 0x07 is disabled BOD. The datasheet of the ATmega328P shows the same BOD level coding.
This means that the default setting of 0x05 for the Uno that came with the IDE is 2.7V. So the Uno already has brown-out detection enabled. That's interesting, because the Arduino Uno runs on a 16MHz external crystal. The datasheet of the ATmega328P shows that at 2.7V the maximum frequency for safe operation is 10MHz. So at 16MHz the minimum voltage for safe operation must be set to 4.3V. So extended fuse must be set to 0x04.
You now learned that it is useful to change the brown-out detection level for your Uno. Don't forget to save the boards.txt file, restart the IDE, burn the bootloader and then burn your sketch. Happy brown-out detecting!