October 2011: Android 4.0.

• For devices without a dedicated menu key, the software compatibility menu key will not be shown even on phones. By targeting Ice Cream Sandwich or later, your UI must always have its own menu UI affordance if needed, on both tablets and phones. The ActionBar will take care of this for you.
• 2d drawing hardware acceleration is now turned on by default. You can use Android.Resource.Attribute.HardwareAccelerated to turn it off if needed, although this is strongly discouraged since it will result in poor performance on larger screen devices.
• The default theme for applications is now the "device default" theme: Android.Resource.Style.ThemeDeviceDefault. This may be the holo dark theme or a different dark theme defined by the specific device. The Android.Resource.Style.ThemeHolo family must not be modified for a device to be considered compatible. Applications that explicitly request a theme from the Holo family will be guaranteed that these themes will not change character within the same platform version. Applications that wish to blend in with the device should use a theme from the Android.Resource.Style.ThemeDeviceDefault family.
• Managed cursors can now throw an exception if you directly close the cursor yourself without stopping the management of it; previously failures would be silently ignored.
• The fadingEdge attribute on views will be ignored (fading edges is no longer a standard part of the UI). A new requiresFadingEdge attribute allows applications to still force fading edges on for special cases.
• Android.Content.Context.BindService(Android.Content.Intent, Android.Content.IServiceConnection, Android.Content.IServiceConnection) will not automatically add in Android.Content.Bind.WaivePriority.
• App Widgets will have standard padding automatically added around them, rather than relying on the padding being baked into the widget itself.
• An exception will be thrown if you try to change the type of a window after it has been added to the window manager. Previously this would result in random incorrect behavior.
• Android.Views.Animations.AnimationSet will parse out the duration, fillBefore, fillAfter, repeatMode, and startOffset XML attributes that are defined.
• Android.App.ActionBar.SetHomeButtonEnabled(bool) is false by default.

Applications targeting this or a later release will get these new changes in behavior:

[Android Documentation]