Nvidia dropped by today to demo some of the awesome things that the GeForce 9400M in the new MacBooks can do that Intel's integrated graphics just can't touch, and to discuss a few technical points. Besides confirming that you'll see it in other notebooks soon, they definitively answered some lingering questions about the chip's capabilities: It can support up to 8GB of RAM. It can do on-the-fly GPU switching. And it can work together with the MacBook Pro's discrete 9600M GT. But it doesn't do any of those things. Yet.
Since the hardware is capable of all of these things, it means that they can all be enabled by a software/firmware/driver update. Whether or not that happens is entirely up to Apple. While you can argue that Hybrid SLI—using both GPUs at once—has a limited, balls-to-the-wall utility, being able to switch between the integrated 9400M and discrete 9600M GT on the fly without logging out would obviously be enormously easier than the current setup, and allow for some more creative automatic energy preferences—discrete when plugged in, integrated on battery. Hell, you can do it in Windows on some machines.
But since it's Apple it's also entirely possible we'll never see any of this to come to pass—GPU-accelerated video decoding has totally been possible with the 8600M GT in the previous-gen MacBook Pros, and well, you know where that stands. [Apple & Nvidia Coverage@Giz]