UFC doesn't release buyrate numbers, so we all usually depend on Meltzer's sources.  Without that, the guessing game is usually based on a few things:  Google search trends, traffic for this website and others, viewers for the countdown show, amount of interest among casual fan friends, and packed nature of bars.

Using these guessing tools, we get a mixed result.  Traffic here and at other major mma websites was down for UFC 83 as compared to UFC 82 and UFC 81.  Google search trends on the event were higher than UFC 82 and 80, but lower than 81.  Google trends aren't everything though, as they were equal between 80 and 82 and 82 did significantly better on PPV.  The countdown show for UFC 83 did more viewers than the prior two, and my casual friends were far more interested in this.  The bar I was at was also the most packed I've seen it since Chuck and Quinton last year.

GSP's status as a draw is a big deal for The UFC.  If he can replace Matt Hughes as a huge Welterweight draw, that will be a big feather in their cap.  My best guess is that this show did very well despite those measures, but who knows.  If the show really did less than UFC 82, it is definitely bad news.  I suspect it'll come in around 500,000 buys, above 82 and below 81.  That would be a good number, considering Couture/Gonzaga did 520,000.