Actually, I’ve just gone and looked at the rules in various parts of the Dope, and this seems to be something of an overstatement.
There is nothing about betting in the Registration Agreement, Rules, and FAQ.
There is also nothing about betting in the Game Room’s rules. (That’s where the post appeared that @Chronos gave a warning to.)
The only places where betting is explicitly forbidden, as far as I can tell, is in Great Debates and in Politics and Elections.
I did not know, until today, that there was any “no betting” rule at the SDMB, and it seems that the rule was never explicitly applied anywhere outside those two forums.