It had nothing to do with not being able to use an Nvidia GPU with Arrandale, and that's still the case with Sandy Bridge. The reason Apple couldn't use a separate GPU in last year's 13'' MBP was because of space constraints, nothing more.
I don't know why they haven't seen fit to include a separate graphics card in the new model, but the most likely reason is that they think the Intel 3000 is up to performing all the tasks the average MBP user would need.
The reason why Apple COULD have put a separate GPU in the new 13'' MBP model is because the entire Intel GPU package for Sandy Bridge is now on a single die. Whereas before an SB chip was required to power the Intel graphics, everything is now contained in a single chip.
So to conclude, there is nothing stopping Apple from using an Nvidia or AMD GPU in the 13'' MBP other than their own preferences. If they wanted to use a more powerful GPU alongside Sandy Bridge it was perfectly possible from a hardware standpoint.
I don't know why they haven't seen fit to include a separate graphics card in the new model, but the most likely reason is that they think the Intel 3000 is up to performing all the tasks the average MBP user would need.
The reason why Apple COULD have put a separate GPU in the new 13'' MBP model is because the entire Intel GPU package for Sandy Bridge is now on a single die. Whereas before an SB chip was required to power the Intel graphics, everything is now contained in a single chip.
So to conclude, there is nothing stopping Apple from using an Nvidia or AMD GPU in the 13'' MBP other than their own preferences. If they wanted to use a more powerful GPU alongside Sandy Bridge it was perfectly possible from a hardware standpoint.