Jacob Berkman of the Jewish Telegraphic Agency, who specializes in reporting on Jewish philanthropy, spent a couple of days on Google with the 2009 Forbes 400, and came up with a list of Jewish Forbes 400 members for use by Jewish charities:
But that's just one man's estimate. For example, he includes Meg Whitman of E-bay, who is running for the GOP nomination for governor of California, but most of what I see about her suggests she is largely Boston Brahmin. And some of his "maybes" on his overall list of 149 possibilities are definitely not Jewish, such as the two Bechtels, who have made billions out of Saudi Arabia. (One of Bechtel's key executives in Ibn Saud's kingdom was Cornelius Stribling "Strib" Snodgrass, whom I'm going to guess wasn't Jewish.)
Note the helpful comments from Santos L. Halper below Berkman's posting for recommended changes, some of which Berkman has gone back and incorporated. And, of course, there are the usual issues of how to count people with one Jewish parent, converts, and so forth.
So, I don't know exactly where Berkman stands at this point. The most accurate number might not be 139 anymore. But it's definitely at least 30% (120), and "about one-third" would appear to be a reasonable approximation.
Part of the increase in Jewish numbers is due to the proliferation of Pritzkers of Chicago (there are now 11 on the Forbes 400). They've been rich a long time (an old friend of mine was their nanny in Europe in the 1970s). Presumably they are multiplying due to inheritance.
But the increasing importance of Wall Street in the economy in recent years no doubt played a major role in this increase over the historic baseline.
If the Forbes 400 is about one-third Jewish, that's less than the one-half Jewish Atlantic 50 of most influential pundits. Tom Friedman's chances of someday being on both lists took a hit this year when his father-in-law Matthew Bucksbaum, who was #205 on the Forbes 400 in 2008, dropped off the list in 2009 when his shopping mall company filed for bankruptcy. He must not have been reading his son-in-law's economics advice books closely enough.