A 'dangerous precedent' looks to be set with the an update to the law in California regarding cellphone usage whilst on the move. Get caught talking on the phone (or texting) and you could get a $20 fine.
Personally I put in a fair few calls on the move, I take a few calls too, I also reply to 'where are you?' texts. Naturally this is 'perfectly safe' to do, but, if I had to look out for Police mid-text then I could come a cropper. A $20 fine would not be a deterrent and 'on the phone whilst cycling' on my otherwise squeaky clean criminal record would be a 'badge of honour'. Hence, for me, I don't think that banning cell-phone use when cycling is a good idea, more of a 'gross infringement of my human rights...'.
Furthermore, if law makers really thought about it they would ban MP3 players and hands-free phones because those can be dangerous when cycling. From that perspective this legislation is on the 'slippery slope'. As it is, if you did cause a multi-car pile-up with scores of innocent car drivers maimed or killed (just because you were on the phone to your mum) the Police could check your mobile phone records and get the proof required to do you for 'reckless riding'.
Although I see such legislation as stinking of 'nanny state' I don't see how best to oppose a ban on cyclists using phones on the move.
Note that this is a subjective question, however, we have had Is listening to music dangerous while cycling? and I would like a few quality thoughts on how best this type of legislation can be nipped in the bud.