NVidia confirmed to Gizmodo that we'll see the 9400M in other notebooks soon as well as definitively answered some 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.
It now remains to be seen whether Apple will released a software update to enable these features. Being able to switch between the integrated 9400M and discrete 9600M GT on the fly without logging out would definitely save a lot of headache.