A lot of people keep mentioning Leopard and 64bit, but Leopard will run just fine on the 32bit hardware too. 64bit isn't going to give Leopard itself any magic speed boost and 64bit software can run under Tiger right now...
But I agree 64bit support is a lot bigger than people think. For applications that are optimized for a 64bit instruction pipeline, the benefits can be huge. I do a lot of 3D rendering and like to use it as an example. 64bit versions of Lightwave (PC only for now, unfortunately) and Maya show speed bumps of 10 to 35 % over the 32bit counterpart software when running on teh same hardware - just the 64bit version being better optimized to use it. Generally speaking, for 32bit apps, C2D is only going to give an average 5 to 10 % speed up, just because it's a bit more efficient and has a larger cache on the faster models that Apple is using.
This is huge... And where I'm sure Apple gets those amazing 39% faster benchmarks for FCP when comparing the new 2.33GHz MBP to the previous 2.16GHz CD model. I can't wait until mine gets here so I can try it out...