Currently, to take advantage of the 2D Hardware Acceleration, each application must have the following in its AndroidManifest.xml:
<activity android:hardwareAccelerated="false" />
Unfortunately, the vast majority of applications have not added that flag. Romain Guy explained why he thought that it was a bad idea to have Android default to enabling the new pipeline, citing custom drawing code as a concern. I'd still like to give it a whirl.
It is not turned on by default for compatibility reasons. Not 100% of the Canvas API is supported when turned on (although the missing parts are very few and rarely used) and there might be bugs in the new implementation. There are also new constraints (for instance if you try to draw a bitmap larger than the maximum OpenGL texture size, it will fail.)
The new rendering pipeline also uses native display lists for each View, which triggers bugs in some apps. For instance, if a View relies on its parent to invalidate() to redraw itself, it's a bug in the app, but it "works" without hardware acceleration. It will however not work with hardware acceleration on.
Our goal is to make hardware acceleration on by default as soon as possible but we do not want to break apps. That said, apps using standard views and APIs should work just fine.