Actually, the percentage of celebraties that walk is about the same as the percentage of 'poor' people who walk. Conviction rates at trial used to be over 90%, the rates have been falling and in some major metropolitan areas are down in the 65% - 70% range. The big differences are (1) the big celeb trials make the papers and (2) 80 to 90% of the 'poor' plea guilty to something and do not go to trial.
The two big celeb trials, OJ and Blakely are especially unique in that the juries were stupid, stupid, stupid. And in the OJ case, at least, the DAs were incredibly inept.
Of course, it was not the jurors fault that they were stupid. They were picked because each one claimed to have no opinion, which meant they had no connection to current events. In the OJ case they also had to be locked up in a hotel for the duration, which meant that the jurors had either no jobs or dead end jobs and no family or social life.
Further, with the massive publicity and cameras all around, the jurors got the idea that they were really in a TV show, such as Law & Order or CSI, where the obvious suspect is never the real criminal and all loose ends are tied up before the credits roll. A real trial, like real life ends up with loose ends, and 99% of the time the obvious suspect did do it.