64-bit ? faster.
As has been said, 64-bit does not directly equate to faster. 64-bit mode means two things:
1. The processor can address more physical memory. In the PPC 970's case, it really only supports 42-bit addressing, but the instruction set (PPC64) supports full 64, and the PPC970 just fakes it. This simply means that a single process can use significantly more memory than a single 32-bit process.
2. The processor can work on data in 64-bit chunks. This does *NOT* magically mean it can do things twice as fast as a 32-bit processor. Indeed, most apps today don't even really need to work on 32 bits of data at a time. (Think about '16-bit' CD-Audio; or '24-bit' images, which are really just three separate 8-bit channels...) Forcing a program to work in 64-bit mode means it has to transfer twice as much data for each piece of data it's working on. If it doesn't really need 64-bits of data, this actually SLOWS IT DOWN, because the processor ends up being a bandwidth hog. The best way I can say it is that a PowerPC 970, with it's 64-bit mode, is, in fact, *NOT* the perfect processor for processing CD audio. A 16-bit processor would be. If the 970 was EXACTLY the same, except with 16-bit data paths instead of 64-bit, it would probably process CD Audio much faster, since there would be no wasted overhead by sending an extra 16 or 48 bits of 'empty' data per data packet.
Try creating a text document, save it in 'Text Only', then in 'Unicode'. Unicode is for support of Asian languages, so it supports many more different 'letters' than plain text (or ASCII,) but that means that each character takes up twice as much space. When you're only typing English characters, you end up 'wasting' this extra space. It's the same thing with 64-bit. If you're not using 64-bit data, you're wasting processor resources if you're using 64-bit mode.
So, (my point finally comes around,) the OS doesn't NEED to be written in 64-bit. Very few things that the OS itself does ever needs to access more than 4GB of memory (again, for the OS itself,) and access 64-bit chunks of data. Likewise with iLife. Unless GarageBand gets a whole lot more powerful with its next revision, none of the iLife apps need 64-bit chunks of data, and none need more than 4GB per process. (Unless you happen to want to record your entire CD library into a single 4+GB AAC file, or store a 38,000-pixel by 38,000-pixel TIFF in iPhoto.)
To put it in perspective, when Apple first introduced the PowerPC architecture in 1995, less than 1% of the OS (7.1.2) was PowerPC native. Most of it was EMULATED. (Which is significantly worse than just being 'the wrong bit depth'.) It wasn't until OS 8.5 in 1998 that the OS was written entirely for the PowerPC.