Q to Chis: When you say "hardware accelerated rendering" do you also mean transforms and bilinear/bicubic interpolation ?
It depends on which direction you are talking about...
If you render to a VolatileImage (implemented as a pbuffer, in hardware), all rendering operations are accelerated via OpenGL, just as if you had been rendering to the screen. Refer to the long list of OGL-accelerated operations that I've posted to these forums from time to time (e.g. compositing, bilinear image transforms, etc, etc).
If you copy a VolatileImage to another accelerated surface (i.e. the screen or another VolatileImage), this copy operation will be handled by OpenGL. In theory, it should be pretty fast, because it's a VRAM->VRAM operation. But in reality, we've found that straight pbuffer->screen copies are not as performant as say, a texture->screen operation. This is especially true when you start enabling per-fragment operations, such as blending, or if you specify a non-identity transform. To answer your question more closely, yes, we will attempt to accelerate pbuffer->screen transforms, but we can only do bilinear in hardware (no bicubic in OGL). These pbuffer->screen transforms will go through an intermediate texture codepath, so performance will not be as good as if you were transforming a managed image to the screen. We're working on a solution for this performance issue.
The bottom line is... If all you want is a translucent sprite, and you want to blend it and transform it to your backbuffer, you should probably just use a managed image. (I mentioned this in the workaround for 5002129.) If you have a complex scene that changes, and you want to render it into an offscreen, then a (translucent) VolatileImage may be better suited for the task.
Now here's an opportunity to ask a question to the esteemed java-gaming.org community... I know many of you were clamoring for the ability to create translucent VolatileImages, and we added new methods in 1.5 for this purpose. The question is: how exactly are you planning to use translucent VolatileImages in your game (e.g. motion parallax style backdrops? simple sprites? something else?)?