Prior to reading this article, I incidentally just finished writing a howto for the recent muxless Intel/ATI hybrid graphics, also known by Powerexpress 4.0 (analogous to Optimus). This can be accessed in the Gentoo Forums at
http://forums.gentoo.org/viewtopic-t-909802.html.
In summary, I was able to get the discrete GPU working with both fglrx (proprietary) and radeon (opensource) drivers on my machine, which is a recent Sony Vaio S 13.3 that has Intel Core i5 2430M CPU and a Radeon 6630M GPU.
The fglrx driver is surprisingly way ahead of Nvidia drivers in that respect. It does in fact interact with the Intel X.org driver to do the rendering on the Radeon GPU. A true hybrid driver. Interestingly, it means that we now have a Intel KMS on the frontend with the Radeon GPU at the backend - a total win. There is a slight lag while using Radeon, but may be its just me.
In the howto, I also wrote about running a bumblebee style second X server on Radeon GPU and using VirtualGL to run 3D applications on this X server. The second X server at the moment can only be run with the opensource radeon driver - no luck with fglrx. So to conclude, fortunately nothing in Bumblebee seems to be actually specific to Optimus, except for the ACPI calls that differ anyway between various systems. I think they should really start including radeon support into Bumblebee.
Overall, it seems there is a little lack of information and even misinformation about the state of radeon hybrid graphics, but I think it is actually going in a good direction. I myself was in a bit of despair with my Sony notebook a week back and currently I am very pleased. All said, I am actually more pleased by the performance and power consumption of Intel graphics on this computer, and hence mostly sticking to the integrated graphics.