When Apple announced iOS 4 details, they explained that not all features of iOS 4 would be compatible on all devices mainly because their hardware couldn't support it. The problem is, they didn't really say much after that as to what features would be left out. It's clear that multitasking will be left out in some older models, but what else? Well, we have that information now so let's let it all out.

iPhone 4 and iPad (2010)
All features of iOS 4 will be available for both of these models.

iPhone 3GS and 3rd-gen iPod Touch (2009)
All features of iOS 4, with the exception of iPhone 4 exclusive features found here, will be available in the iPhone 3GS and 3rd-generation iPod Touch models in 32GB and 64GB. The 8GB 3rd-gen iPod Touch compatibility information falls under the category of the 2nd-gen below.

iPhone 3G and 2nd-gen iPod Touch (2008)
The following features of iOS 4 are not available in these models as well as the 3rd-gen 8GB iPod Touch:
- Multitasking
- Rotation lock
- Custom wallpaper
- Bluetooth wireless keyboard support

iPhone and 1st-gen iPod Touch (2007)
iOS 4 will not be available for either of these models for updating.

What strikes me as odd is the fact that a custom wallpaper isn't available on the iPhone 3G and iPod Touch second generation. Apple claims hardware support is the main issue, but does a custom wallpaper really eat up performance or battery life? I can see animated wallpapers like on Android, but something's fishy about the absence of a custom wallpaper for these models.

Update (6/20): There's now rising speculation that the iPhone 3G will support custom wallpapers due to the custom wallpaper on the icon for the iPhone 3G on Apple's iOS 4 webpage. Take a look:

So check above to make sure you have all the details necessary for updating your iDevice on Monday when iOS 4 is released for FREE on both iPhone and iPod Touch. iPad support will be available later this summer.