Unbelievable how much misinformation is being bandied about in this thread. For example, the "loophole" everyone is referring to is clearly spelled out in the rule- no lawyerly digging or super-secret knowledge required. The conditon allowing the pre-1990 grooves applies to all pre-1990 clubs manufactured with grooves that conformed at the time, not just Pings. The Tour did not decide that certain grooves are "not part of the game." The new rule has nothing to do with the legality of square grooves- they always have been, are, and presumably always will be legal. The USGA knew exactly what it was doing when it revamped the groove rule, as did the PGA Tour when it decided to implement the rule as a condition of its competitions in 2010. Due to the two settlements of the Ping legal actions in 1988 and 1993 with the USGA and the Tour respectively, not grandfathering these clubs was not a possibility. However, no organization has ever proved a perfomance benefit using those older grooves. The new regulations were a response to the extra sharp/aggressive "spin milled" grooves developed over the past ten years or so, not those of twenty five years ago.
The new regs say in essence that in clubs with more than 25 deg. loft, square, U, box, whatever, must be designed so that the spin generated in tests stipulated by the USGA is the same as traditional V grooves. One can access all the research and notices about the grooves rule as well as the new condition itself at USGA.org
McCarron was clearly out of order in calling out Mickelson in the manner he did. He could have handles it in private, or at least expressed his reservations without calling out a fellow tour player. However, Phil missed a real chance here. By using clubs that conform to the new rules and playing well or winning, he could have set himself up as the poster boy for tour ethics, as opposed to, say, Daly. Now he is not only not that, he seems on the verge of suing a fellow tour member, and the distracton has apparently caused him to bomb out of this weekend's tournament.