While we're still wondering why Apple would keep the latest Arrandale chips away from its 13-inch professional laptop, the company enlisted Nvidia to make a custom graphics part.
Nvidia told AppleInsider that the 320M integrated graphics part was made especially for Apple as a suitable replacement for the 9400M in the 13-inch MacBook Pro. While the 320M isn't as capable as the GT 330M in the bigger MacBook Pros and other PC laptops, the 320M is a notable improvement over the 9400M.
Apple boasts that the 320M is up to 80 percent faster than the 9400M in Doom 3, while in other games, such as Quake 4, the gains are just 30 percent. The 320M is supposedly based on the GT216 core, which gives it 48 shader cores. While the spec sheet does boast 256MB, it is shared with the main system memory.
Clearly, the 320M isn't really going to play Crysis with any satisfactory results, so the real achievement here is that the 320M is 40 percent more energy efficient over the previous IGP while also being faster.
What's interesting is that Apple's choice to stick with Core 2 Duo CPUs, and thus been able to keep Nvidia on-board for the chipset. Perhaps the conditions so happened to be that Apple would stick with Nvidia for its 13-inch MacBook Pro offering as long as it would get a custom IGP, even if it meant not taking the leap to Nehalem-based Core iCPUs.