Tallest Skil, You sure do like promoting the idea of buy now! if there isn't a genuine need or compulsion, no issues in waiting as long as you know what you're waiting for.
That said, if the only thing you're waiting for is 10.6 - i think you may be misguided. If you're looking for hardware to take advantage of Grand Central or OpenCL, it's already here. Any 8600 or 9600 or 9400 GPU is up for the challenge.
There will be two revisions next year. One will be a speed bump early on (possible graphics update, but highly unlikely), i'd guess at end of first quarter, but it depends how long apple wants to sit on discounted chips. The second will be the calpella (nehalem) upadate, likely towards the end of the year around october/november. Which will be a significant and something that is worth waiting for, if you can wait that long.
Buying now, as darwinian said is a safe enough bet and represents decent value if you're buying with US$, while it is Rev.A, many of the early manufacturing bugs appear to be worked out.
Need is a very vague idea, basically if you're going to use it a lot from today until march next year, buy today. If you'll be taking some nice christmas holidays, maybe wait until the next revision. If you just have to plan your purchases around the 'best time to buy' because you're obsessive, like so many, calpella is where it's at.
simple, non?