I believe when Apple made the transition to Intel processors, they made some comment in passing that the first gen would once again only be 32bit but would change fairly quickly. They went from making a big deal about G5s being 64bit ready to not mentioning anything any where about it and then once 64bit intels were available, they started talking about it again. I could be wrong about that but it is what I vaguely remember reading about.
bits are the bus lanes that apps use. So when you have 32bit apps they only utilize 32 of a possible 64 channels that take data from RAM to processor. utilising all 64 bits allows apps to theoretically run faster as more data is pushed through the processor cycle at any given time.