Hi, Couple of months ago, I bought my Arduino Duemilanove Board, as well as an "Extra" ATmega328 chip. My problem was: I didn't notice that the seller burned the bootloader for arduino uno. I knew when you put the chip into the board and press the reset button, the led on the board will blink ONLY once! if it has the correct bootloader, otherwise the correct bootloader must be burned. So I put my extra chip onto the duemilanove board and I pressed the reset button, everytime I press the reset button, the LED will blink twice or thrice. So I assumed that when you burned the bootloader using Arduino UNO as ISP or burned the bootloader of uno into a chip and you use such chip for duemilanove, the IDE won't program any codes to your chip.
Am I right?
Anyway, after so many readings, I've got a way to burn the correct bootloader into my extra chip (internal clock) just using the duemilanove board and a breadboard. BUT, suddenly, I lost my extra chip. 
and I can barely afford to buy one.
I want to know:
- My assumptions and theories are right, right?
- Is there anyway for me to get the codes from my Atmega chip? ( no right? What the heck was I thinking? ). I already programmed my codes into my original chip and I accidentally deleted my source codes. damn.
- I can't burn the bootloader into my extra chip (if I will buy tomorrow) using Arduino as isp WITHOUT PROGRAMMING THE "arduinoISP" program (from File>>Examples>>arduinoISP) into the original chip which is attached on to the board, can I? =(
Most of your questions are too vague or confusing. However your title question is pretty simple to answer:
Bootloader for uno and duemilanove: are they different?
Yes they are different:
Uno bootloader is smaller takes up less flash memory space
Uno bootloader uploades sketches at a faster baud rate
Uno bootloader handles WDT interrupts correctly
They are similar in that:
Both will run on a 328p chip.
If you load a Uno bootloader onto a duemilanove board it will work but you will have to forever select Uno as board type to upload sketches to it. With a Uno bootloader the board becomes a Uno board.
If you load a duemilanove bootloader onto a Uno board it will work but you will have to forever select Duemilanove as the board type to upload sketches to it. With a duemilanove bootloader the board becomes a duemilanove board.
Lefty
wow geez. thanks for your reply!
Okay, let me finalize, so you mean: If I buy a new chip (note: this has an UNO BOOTLOADER), and I attach it into my duemilanove board, I can compile say, the BLINK example, right? I JUST HAVE TO SELECT: TOOLS>>BOARD>>ARDUINO UNO
Am I correct? If yes, then, ughh, Im so stupid. haha. But anyway, I didn't waste anytime searching for the right way to burn the bootloader.
glenn_boy13:
wow geez. thanks for your reply!
Okay, let me finalize, so you mean: If I buy a new chip (note: this has an UNO BOOTLOADER), and I attach it into my duemilanove board, I can compile say, the BLINK example, right? I JUST HAVE TO SELECT: TOOLS>>BOARD>>ARDUINO UNO
Am I correct? If yes, then, ughh, Im so stupid. haha. But anyway, I didn't waste anytime searching for the right way to burn the bootloader.
Your are correct, but most likely not stupid. 
It's difficult at times to define what makes a specific arduino board different from other boards without a clear context of what is being attempted. For instance an arduino Uno and a arduino duemilanove will always have to use different PC USB serial software drivers regardless of any change to the bootloader type installed on the 328p chip, because the type of USB serial converter hardware is different and will always be different from each other, but that is a different context of how and what the arduino IDE needs to know to successfully upload a sketch to an attached board.
Lefty
I've got a chip on a breadboard that thinks it's a Uno, because I put the Uno bootloader on it:
I've also put the Uno bootloader on my Duemilanove. So it works fine.
However you could be pardoned for being confused about what a Uno really is.
thanks sir nick. I lost my another chip so I had to buy a new one. This new one has the UNO bootloader, and my board is duemilanove. I have my program in my another chip and I lost the source code too. That's why I should not use it to burn the duemilanove bootloader to my new chip.
anyways, like the another guy said, you can use the new chip. If it has the UNO bootloader, and you have different board, just choose BOARD>>UNO. I tried it! 
No errors. 