For as celebrity obsessed as our cultures tend to be, this was actually a pretty hard statistic to confirm or debunk. It just seems that very few people kept track of it in actual numbers. That may be due to the fact that how does one exactly classify someone as a "celebrity".
I did find one article that seems to pop up in all my searches that said:
The United States has the world's highest marriage rate of 9.8 per 1000 as well as the world's highest divorce rate of 4.95 per 1000.
A recent study done on Golden Globe award winners revealed that of the celebrities who had been married, the average number of marriages was 1.4, while the average number of divorces was 0.65. The probability of a famous union surviving is only 35 percent, so the odds are 1.9 to 1 against the marriage.
This site (which I can't say much about) also repeats this study without actually linking to it or referencing it properly:
Well, because we thought you had a right to know, we conducted a comprehensive first-of-its-kind study of celebrity divorce. The study focused on Golden Globe award winners, who should fairly represent celebrities as a whole. (The Golden Globe categories include both television and motion pictures.) All 2001 and 2000 Golden Globe nominees and winners in the acting categories were included in the database, for a total of 151; due to multiple nominations and awards, the final yield was 127 unique celebrities.
Of the 127 celebrities listed, 68 percent had been married at least once. Billy Bob Thornton takes the honors of having the most marriages with five (as well as the most divorces, also with five of course, by law it had to be at least four divorces, but you never know with Billy Bob).
Of the celebrities who had been married, the average number of marriages was 1.4, while the average number of divorces was 0.65. The probability of a celebrity marriage surviving is only 35 percent, so the odds are 1.9 to 1 against the marriage.
So, from this unverified "study", it would seem that the rates for celebrities is indeed higher than the general population.
The only reason I even decided to answer this was because of a blog post by a blog I am following that popped up today. In that post, they mention a group called the Barna group doing a study on US divorce rates, and they seem a bit lower than 50%, but that was from 1999. Even so, a website dedicated to divorce rates also seems to back up a rate at less than 50%.
The CDC tracks it a bit differently, by giving it a stat like: Marriage rate: 6.8 per 1,000 total population, and Divorce rate: 3.4 per 1,000 population. In this case, it seems that the 50% number holds up. (Although the marriage and divorce rate is different than the rate claimed in the first article, so that would cause me to question that article a bit more.)
BOTTOM LINE: Yes, it appears that celebrities do have a higher divorce rate than the average person (dependent on how you define celebrity of course).