http://www.xkcd.com
Anyway, time to post a long thread to clear up some misconceptions floating around about dual graphics....
1. Q: Does the MBP use both graphics cards at once?
A: Not really. When the 9600 is enabled, both cards will appear as separate OpenCL devices. In theory, an app could be written that targets both separately. Writing an app to do this, however, would likely be a lot of extra work that doesn't see much benefit on most machines.
2. Q: Does it switch on the fly?
A: No. Some apps don't like having a more powerful GPU switched out from under them. Apple likely made it so you have to log out so that the user wouldn't have to worry about such things.
3. Q: Nvidia has a technology called "HybridSLI". I've heard the name, but I don't know much about it. What is it?
A: HybridSLI is a blanket term referring to two things: GeForce Boost, which allows the IGP and discrete graphics to be used as an SLI pair for extra performance, and HybridPower, which allows the system to toggle from one card to another for extra performance.
4. Q: Does the MBP support HybridPower?
A: Not exactly, see #2 for the likely explanation.
5. Q: Does the MBP support GeForce Boost?
A: No, because the 9600 itself doesn't.
6. Q: Huh? Why not?
A: Most likely, it is due to SLI scaling. The 9600 would have to downclock, and that combined with the bandwidth between the two chips and the 9400's poorer memory bandwidth probably results in very little performance gain over just the 9600 by itself.
7. Q: This Dell over here supports HybridSLI!
A: That is because the cards involved are different (lower end).
8. Q: Will the MBP ever be able to switch on the fly?
A: Assuming #2 is the actual reason for the log out, I'd guess "no".
9. Q: Will the MBP ever be able to use both cards at once for graphics, or show them as a single OpenCL device?
A. No, the 9600 doesn't support it in hardware, end of story.
10. Q: Any further reading?
A: Check out this page: http://www.nvidia.com/object/hybridsli_notebook.html Be sure to read the Tech Brief PDF in the sidebar there too, if you'd like to learn more.