I'm not particularly familiar with multiple processors in pre-OS X, but here's my question. If you are multitasking, and have two separate, intensive programs running, wouldn't Mac OS automatically assign each program to its own processor? So even if the apps themselves aren't multithreaded, running more than one at a time could see a considerable benefit?
Or did it not work that way so long ago?

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I am not yet quite to the bottom of this, but it is a good question and it appears that the answer is that it did not work that way, unfortunately. There is only one primary processor per machine, to which applications are in the first instance assigned. Applications (e.g., Photoshop) which are then MP aware can use the second processor as well. That, of course, is also true in OSX (not sure about Snow Leopard), except that in X, the OS can allocate one program to one processor and another program to another processor. I think....

However, it appears that some of the operations of OS 8.6 itself are using both processors, as I get partial use from the second processor just from, say, opening a file folder. I do not, however, yet have a good way to measure how this works in action, beyond some fragmentary information provided by one of the (very basic) apps in the stock Multiprocessing Folder, which tell me that the second processor is working, say, at 15% or some such.

Not ideal, but it still feels snappy.