Perhaps rule #216:

http://www.nhtsa.dot.gov/cars/rules/rulings/RoofCrushNotice/216NPRM-to-FR.html
http://www.autosafety.org/article.php?did=1183&scid=174

I didn't have time earlier to expand, if that rule is indeed the one they're referring to, I would guess that the cars in question are the Pontiac Solstice convertible (perhaps the Corvette convertible as well) and the Honda S2000 convertible

__________________
Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE