What to do with more than 32 bits of address space? Hmm, the first thing to note is the difference between the size of an address space and how much memory you physically have in your machine. The BSD family (and others?) hve for a while offered the ability to "memory map files". This is an area where more than 32 bits of address space is relevant, but without needing more than 4GB of memory. Let me explain more....
The traditional approach to manipulating files is to have a "current file pointer" (or offset from the start of the file) and to perform read or write operations from this pointer to memory buffers dynamically allocated to hold data. The change to memory mapped files proposes a significantly different approach to accessing the contents of a file. In essence, using a memory mapped file involves asking the OS to provide a region (PTR) of memory that corresponds to the file. Reading (PTR+N) in memory reads byte N from the file. Writing (PTR+N) in memory writes byte N in the file. Under this model, leaping all over the file is easy, directly mapping byte sequences in the file to accessiblle data stuctures is easy and all the dynamic memory allocation within the application is eliminated - this is changed to the OS making the decision of what (how much) memory is used to cache file contents. Not only is this generally more efficient, it makes it practical to easily manipulate files over 4GB without any effort.
So i think the first real use of a 64 bit processor is not being able to put more than 4GB of memory in the machine - rather, it is the easy ability to efficiently work with very large files (think raw video files, for example). This may not be immediatelly useful to all Mac users, but for the creative video community, this will provide a range of enhanced software over time.
There are also other more esoteric applications of such large address spaces. For example, the 64 bits of address can represent 40 bits of address and 24 bits of "tag type" indicating what sort of data is contained at that address. For polymorphic interpreted languages, this can provided major improvements - although esoteric as I said.
A single DVD side/layer is about 4.7GB. A DVD can be double side and double layered, giving 4x4.7GB. A 64 bit address space means there is nothing alarming about this quantity of data. A 32 bit address space requires massive fudges to handle such quanitites of data.
But...I still want one 
