When Apple released iOS 5, almost all the users saw it's welcome background and we all loved it even me, anyway you can download iOS 5 background on your iPhone/iPod Touch and iPad.
First before I tell you anything, I will give you below three links one for iPod Touch onther for iPhone and iPad.
Download iOS 5 Welcome background for iPhone.
Download iOS 5 welcome background for iPod Touch.
Download iOS 5 welcome background for iPad.
Once downloaded unzip the file and import the photos on iPhone via iTunes or transfer it via email. Once saved on your iPhone, you do not have to do is select it from the roll and set it as wallpaper.