45nm is the size of the process technology Intel uses to make their chips, usually referred to as a "die-shrink". Basically, 45nm is the size of the smallest components on the chips. This decrease in physical size allows the chip to have a higher transistor density, use less energy, be more energy efficient per mhz, etc. The decreased power consumption allows the chips to run at a higher clock speed and consume the same amount of energy or keep/lower the clock speed and gain better battery life for a laptop.
Right now Intel has three groups of Core 2 mobile processors.
The standard Core2 Duo mobile processors which have a power rating of ~34W. There are also two categories of lower voltage chips which run much slower and cooler, but they are not used in most laptops except for small subnotebooks. (Intel Core2 Duo Low voltage (LV) and Ultra-Low Voltage (ULV)
The new "Penryn" standard line of processors will run at 35W and 29W.
In addition to the two low voltage lines, there is a new category of "medium-voltage" processors which have a 25W TDP, but which retain most of the speed, cache size, FSB speed, etc.. Which means they may show up standard laptops. I think these will be the best fit for a subnotebook or long running macbook.
In addition to the die-shrink, Penryn brings many new enhancements to the chip architecture itself, including SSE4 which will speed up multimedia-type operations (video encoding, audio encoding, rendering) for applications that are updated for it.
and the desktop Penryn's are already out. Way to go Apple. 
