Battery backup

It seems to me, and I could be wrong, that the arduino automatically switches from usb power to wall wart.
Why not leave 2 power pins on the board from the usb so I can plug in a battery backup.
Am I missing something here, or does this sounds too simple.?

“the arduino automatically switches from usb power to wall wart”

Depends on the Arduino. Uno, Duemilanove, yes.
Promini, no.
Others, need to confirm on schematics.

Most boards have a 5V or Vcc pin where you can bring in external regulated source, or a battery.

Ok its true, they are not the same, but from what I understand, the UNO switches over at 7.5volts (I'm not even sure of this) so if you have a 6volt battery backup bobs your uncle. Vcc XYZ ABC it all sounds complicated, and you would have to look at the schematic's, get a job at Intel to figure that out, I always liked the the KISS thing, keep it simple lots of beginners out there.

Uno is designed to switch when Vin/2 is > 3.3V

Why not leave 2 power pins on the board from the usb so I can plug in a battery backup. Am I missing something here, or does this sounds too simple.?

Just take an old spare USB cable and lop off the computer end connector, id the + and - USB power wires and wire that to your +5vdc battery source and plug it into the arduino USB connector. If Vin external power goes away the battery will pick up the load.

Lefty

Battery backup - 3 AAs, 3 Cs, 4.5V, connect to 5V & Gnd on the Power Header. That's pretty simple.