As Apple is currently the only GeForce 9400M-using company with systems available the amount of applications available to show off the performance of nVidia's 9400M was rather limited, but Call of Duty 4 and Photoshop CS4 were on hand to play with both applications also loaded on an Intel G45-based machine (a Sony Vaio FW as it happens). Running CoD4 at 1280 x 800 with everything set to medium the 13.3in MacBook was definitely what I would call playable, which is a lot more than can be said for the Intel system. The frame rate never really dipped below 25fps, except when walking through smoke - and I'm sure a bit more tweaking of the in-game settings would sort that.
The higher, 1,440 x 900 resolution of the MacBook Pro proved more of a challenge and the 9400M wasn't quite powerful enough to provide a decent frame rate. Luckily the system supports nVidia's Hybrid SLI, whereby the integrated GPU can be coupled with a dedicated chip to provide better performance. This switching does require a log out on Macs (although in Windows no such problem exists) but either way is definitely preferable to no switchable graphics option at all.
Combining the GeForce 9600 with the GeForce 9400 makes it possible to play CoD, and by extension I expect any other game, on the MacBook Pro at its native resolution with smooth frame rates. Of course it's entirely possible to bundle a dedicated GPU with an Intel chipset and get pretty much the same results, so at the high end there's arguably a less compelling case for nVidia over Intel.
As proved by the Sony Vaio Z-series, nVidia is quite happy to offer switchable GPU options with Intel solutions, too, so even the power saving benefits of a hybrid solution aren't limited to nVidia plus nVidia. Although I maintain that at the purely-integrated end of the market nVidia's solution is much more tempting.
Loading up a 3GB (yes, that's three gigabytes) file in Photoshop CS4 the benefits of GPU acceleration are immediately obvious. Even with an action as simple as zooming, there is a clear benefit, with the action being significantly smoother - my ineptitude at using the new large Multi-Touch touchpad come mouse button sported by the latest generation Macs notwithstanding.
From the looks of it, some of the filters either still need to be updated to run on the GPU, or require more than a 16-core chip to really see any benefit. Adobe is pretty good at chucking out updates, though, and there is of course a big third party development community out there ready and willing to fill the gaps.
Perhaps the most important thing nVidia has done is (supposedly - nobody will talk figures due to confidentiality clauses) manage to offer its own chipset without a premium over Intel's graphically inferior part. Centrino, and now Centrino 2 may have a decent foothold in the market, but as Apple adoption of an nVidia chipset proves, there's definitely an opportunity for nVidia to take a chunk of traditionally Intel market share.
If it means cheaper, longer lasting, faster systems for us consumers, then what's to complain about?