What's the first thing that pops into your mind when you hear the phrase Grand Central? If you're from New York City, chances are it's either Grand Central Terminal, or the subway station that adjoins it. Wikipedia has a disambiguation page for "Grand Central" that's been updated this week to include a reference to a new usage, but so far the article pointed to is just a stub, containing a one sentence description: "Grand Central is a technology developed by Apple to optimize multicore application support in Mac OS X 10.6."
In a press release, Apple says this: "Snow Leopard delivers unrivaled support for multi-core processors with a new technology code-named “Grand Central,” making it easy for developers to create programs that take full advantage of the power of multi-core Macs."
But why name it Grand Central, unless there's metaphoric value in stirring up an association with big, busy transportation hubs? (I'd rather not get more specific than that, as I suspect it would be easy to attribute too much significance to the detailed design and operation of one particular hub, looking for clues that aren't there.)
In a transportation hub, conveyances of one sort or another enter, let off and board passengers, and exit, and people enter and board a conveyance, transfer between conveyances, or get off and exit. A computer runs processes, which may themselves be divided into threads, and which are commonly separated between system space and user space. It has various resources which processes/threads make use of. Managing these processes/threads and resources is a much more complicated task than managing the trains and other conveyances passing through a transportation hub (busy airports possibly excepted), although it seems likely that a system software component vaguely analogous to such a hub might help to simplify the problem, and to judge by the sparse information already publicly available that would seem to be a major part of what Apple has created.
Just as conveyances interact with the infrastructure they depend on (sensors, traffic signals, drivers who obey them, ...), processes/thread which are designed to cooperate with the management system can play a role in making things run smoothly, and it seems Apple has also created tools to facilitate writing software that plays nicely with the system.
Whether this interpretation is on the right track, and what the details might be, remain to be seen. We should know in about a year, if not sooner.