As noted recently by Chipworks, Apple is now shipping some iPad 2 models (and all third-generation Apple TV devices) with a 32nm A5 processor built on Samsung's power-efficient "high-dielectric metal gate" (HK+MG) process. These products give Apple a relatively low-volume test bed to ensure its architecture works well with the process, and extensive testing by AnandTech shows that the 32nm process offers significant power savings over the older 45nm process used for processors in other iOS devices.
Apple's A4, A5, and A5X processors, used in the most recent iPhone, iPad, and iPod touch models, are built using the 45nm process. Effectively, 45nm is the absolute smallest size that can be used to create a transistor or other circuit element on a chip using a 45nm process. This also affects the minimum distance between elements as well.