It changes often and I can't be bothered posting it, but right now... I've got a really colourful Osmos screenshot behind my Apps.
The layout remains the same as my 1st Gen IPT waaaay back in Sept. 2007! With the addition of course of extra native apps, which are also in the same order, and my 3 most used apps, Facebook, IM+ and Hotmail. (I hate change you see!) And a classic, home-made background of Pokemon Blue
Yeah go to this link on your device and hold the image to save it to your camera roll: http://krapps.com/wp-content/uploads/2010/07/PacManiPhoneWallpaper11.png
Thanks for that image, Zincuous - one of the cooler ones I've seen so far! I'd use it if I could... Here's my very boring before and after iOS4 home screens:
I see iShred, Isotope, Fastar, Minigore, Catacombs, Helsing's Fire, Angry Birds, Geared, Implode, iBlast Moki and Crossfingers in there. Surprisingly fun trying to see how many icons I recognize.
I get mine here: http://www.blog4iphone.com/wallpapers/ They've got everything from celebs to nature to abstract.
@Mindfield I might check it out soon, I use the stock alarm to its fullest but it lacks a lot and the sounds are aggrivating. I also tried White Noise recently and it's meh, the fade in on the alarm function is nice but that's about it. Here's my app page