Arduino releases 5V from 9V battery?

I'm a beginner to all this so this might be a very simple question.

So far I've been powering my arduino duemilanove through the USB. I just got a 9V battery connector. I know the Power pin says 5V, but since it will be connected to a 9V battery, will it release 9V instead? Or will it automatically downgrade it to 5V?
(I dont want my LED's to blow up!)

if you connect the power to the power jack, the 5volt pin will still supply 5 volts, and the vin pin will supply 9 volts.

all the digital pins will also be at 5v if you turn them high as well. The 9v batt goes through a 5v regulator before it powers the chip

Do not connect the 9V battery to the 5V pin. connect it to the 9V pin, or to the coax power jack and the onboard regulator will take care of the rest.

-j

I was I bit confused by the replies, so I looked up the schematics: http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf

You can see that 5V pin is connected to 5V after the regulator, and Vin is connected to connection 2 of the round power socket (this would be 9V). Thus putting 9V in the power round socket or 9V in Vin pin is exactly the same thing.

It seems that if I have a 5V DC supply, I can feed the board by feeding 5V to the 5V pin (Question: is it safe to put 4 AA rechargeable batteries into this pin? -that would be 5V)

is it safe to put 4 AA rechargeable batteries into this pin? -that would be 5V

No, that would be approximately 5V. When the batteries are freshly charged it may be over 5V (which could kill ICs), and after they've been used for a while it will be less than 5v (which will cause erratic ADC readings, and could corrupt the bootloader on your Arduino if you try to upload a sketch while it is underpowered).

I wouldn't feed anything other than a regulated 5V supply to the 5V input and nothing less than 7.6VDC to the 9V input to get safe, sane, and reliable power to the Arduino and any devices connected to it.

-j

Correct. The voltage may fluctuate too much. Found a nice DC-DC converter that uses 2 AA batteries: http://www.bodhilabs.com/info.html#APPS

Found a nice DC-DC converter

These are appearing all over as "portable cell phone booster/charger" and MP3 battery extenders. Some kits, some nicely packaged consumer units. Nice little modules with either room for AA batteries, or a built-in rechargable, and typically with a USB port for the output.