It's actually enabled by default on Android 4.0. It's optional only in Honeycomb.

It depends on your targetSdk. It can't be enabled by default because it breaks a lot of apps.

You're currently downvoted, but you're correct. Apps targetting older sdks will need to opt-in to make use of the acceleration, even on ICS. But it's an easy switch to flip, assuming it doesn't cause visual glitches, which it can in some cases.

Of course, they would have made it a default if it didn't have some incompatibilities; and looking into it a little more deeply, it does look like there may be:

http://android-developers.blogspot.com/2011/03/android-30-ha...

Well hopefully once the issues are worked out, future versions will be opt-out instead.

The opt-in is basically saying, that either recommended or minimum OS version for your app is Ice Cream Sandwich. You do not switch the acceleration on or off, you say in the manifest, that you are aware of the platform.

There's also an override in the Dev menu to force GPU acceleration for all apps. It also strangely makes the emulator significantly smoother.