It all comes down to chip count, power draw and profit.
Prior to the introduction of the 9400m-based MacBook Pros, standard Macbook motherboards had 3 main chips - the CPU itself, the north bridge, which incorporated the memory controller and Intel graphics, and the south bridge, which handled I/O and various other things.
Macbook Pros, prior to the 9400m, contained 4 main chips - the 3 above (north bridge sans integrated graphics), plus an additional discrete graphics chip.
What made the 9400m special, from a system-builders point of view, is that, in one chip, it provided graphics, north bridge and south bridge functions. This allowed the standard Macbook to go to a 2-chip solution - Intel CPU plus nVidia 9400m, while the Pros, with discrete graphics, went to a 3-chip solution.
From Intel's perspective you can see the business issue - you go from supplying 3/3 or 3/4 of the main chips in the machine, to 1/2 or 1/3.
With the move to Arrandale, the need for a chip to perform north bridge functions (i.e. memory controller) has disappeared, as this is now included within the CPU. Were Apple to include Arrandale in the 13 inch model, and not rely on Intel graphics, they would need a 3-chip solution - CPU, south bridge, and graphics chip, just like the new MacBook Pros. Obviously, for reasons of cost and perhaps power draw, they haven't gone this way, and it seems with the 320m they have a new version of the 9400m, allowing for a Core 2 Duo 2-chip solution.
Why can't nVidia bake an Arrandale-compatible Southbridge into its graphics chip and provide Apple with a 2-chip solution? Because Intel won't grant them the legal ability to do so.
The results? In the new Macbook Pro 15 and 17 inchers, Intel supplies 2 of the 3 main chips, up from 1 out of 3, while Apple has refused to concede defeat with the 13 inch Pro. Clearly a business victory for Intel.
