California's "hands-free" cell phone law had only been in effect a few hours before police across the state began setting up checkpoints to nab gabbers.
I would doubt these checkpoints are legal. The Supreme Court has approved checkpoints to find drunk drivers and fugitives and to catch illegal immigrants or drug traffickers near the border, but has generally said they're unconstitutional for other purposes—including, believe it or not, for random drug searches.
MORE: Per the comments, the newscast may have been misleading in calling these "checkpoints." If it's just a matter of police watching from the side of the road as people drive by, it's not a checkpoint. Which, come to think of it, is almost certainly what's happening. If these were actual checkpoints, you'd think most people would have the common sense to hang up as they approach.