If you catch up to them at a stoplight, here are two pretty good options IMO:
~Whip out your phone and take a picture of them. People tend to get self-conscious being filmed while breaking the law.
~If you have sunblock on your arms, rub your left elbow on their rearview mirror. (I've done this twice and it was most satisfying; that paste sunscreen is a complete PITA to clean off.)

As for all this concern about getting shot, if they've got their phone in their hand, you'll probably have time enough to evade. (Texas, Saharizona, and Floriduh excluded, as they're probably driving with their gun in their non-phone hand.)

Edit: Also, as long as we're fantasizing about worst-case scenarios, their sentence for shooting an unarmed cyclist will be a lot harsher than if they run you down while texting. (Whoopsie. Accidents will happen.. Please don't do that again.)