There are a few caveats:
- Not all features are compatible with all devices. For example, multitasking, custom wallpaper, and Bluetooth keyboard support are available only with iPhone 4, iPhone 3GS, and the third-generation iPod touch (32GB and 64GB models from late 2009).
- Multitasking is available with apps that have been developed to work with iOS 4.
- Books are available on the iBookstore. Title availability is subject to change.
- Faces works only with iPhoto on the Mac.
- iPod touch 2nd generation (late 2008) - but you won't get multitasking or some of the other features
- iPod touch 3rd generation 32GB and 64GB (late 2009)