In the review process for top conferences, conventional wisdom says that rebuttals don't usually change the mind of the reviewer... they only help the authors feel better. The latter is certainly true: after writing a rebuttal, I always feel like I have a plan to sculpt the paper into great shape.
Here's some data from a recent PC meeting about whether rebuttals help. For the MICRO 2011 review process, reviewers entered an initial score for overall merit and a final score after reading rebuttals and other reviews. While many factors (for example, new information at the PC meeting) may influence a difference in the two scores, I'll assume that the rebuttal is the dominant factor. I was able to confirm this for the 16 papers that I reviewed. Of the roughly 80 reviews for those papers, 14 had different initial and final scores (there were an equal number of upward and downward corrections). In 11 of the 14 cases, the change in score was prompted by the quality of the rebuttal. In the other 3 cases, one of the reviewers convinced the others that the paper had merit.
For 25 randomly selected papers that were accepted (roughly 125 reviews), a total of 14 reviews had different initial and final scores. In 10 of the 14 cases, the final score was higher than the initial score.
For 25 papers that were discussed but rejected, a total of 19 reviews had different initial and final scores. In 14 of the 19 cases, the final score was lower than the initial score.
It appears that rebuttals do play a non-trivial role in a paper's outcome, or at least a larger role than I would have expected.