Good things come to those who wait. Memorem will be in the MacBook Pro shortly if Apple wishes to remain competitive. I'd expect somewhat of a quiet update?
The biggest diferece between 64-bits and 32 is that with 64 bits you can use a lot more RAM. The number of bits refes to the lenght of an address. In theory with 32 bits the maxium addres in RAM can be 2 to the 32nd power wit 64 bits it is 2 to the 64th power. In real life Apple terms 32 computers max out at 2GB while 64 bit computers max out at either 16GB or 32GB. In the future 64 bits computers could address terabytes of RAM while 32 bit machines will be limited always to "only" 4GB.
I'm waiting for thr 64-bit imac, because yes, I would load up the thing with 4GB or RAM or even more. One reason for neededig so much RAM is VMWare or Parallels. There virtual environments require that yuo have enough RAM for each VM you run.
I'd allo buy a memron powered 64-bit "mini mac pro" short tower if Apple sold one.
Another one The upgrade wont be massive but obviously an upgrade. I have no patience and dont need the extra power so I have bought one now. But if you need it then wait or if you dont want to wait just buy it now! The MBP you'd buy now will still be great