I think parts of it will run 64 bit, if you have a 64 bit processor, while other parts of the operating system only run 32bit..
Yes and no, if you have a 64-bit processor you will be running the 64-bit version of OS X. However, your exisiting programs will most likely be 32-bit (think XP x64) so you'll be running a 64-Bit OS and your applications will be running in an emulated 32-bit environment.
That being said, emulating a 32-bit environment should not affect performance because the OS will only feed the processor instructions 32 bits at a time.
Conversely, if you have a 32-bit proc you'll install the 32bit version of Panther and your limited to 32-bit programs.
Buying a 64-bit processor is like buying an HD-DVD player for use on your CRT TV, sure its higher res, but your limited by your TV. Similarly your 64-bit OS will be (99% of the time) running 32-bit apps.
Programmers must code for the masses, right now the masses are running 32-bit, therefore writing 64-bit code is unfeasible from a marketing point of view, why cut out approx. 70% of your market by writing 64-bit code.
My prediction is that 64-bit wont be "essential" until mid 2008.