This seems to be on the rise. We've had two events in the last week where moped riders have either mounted the pavement or almost ridden down someone crossing the road and stolen the phone out of their hand.
If you are in Central London with a mobile in your hand, just take a bit of extra care and look around. Especially at mopeds. Don't walk down the street looking up stuff on it, stop and keep a watch around you.
Sadly if they try it on me and I grab and throw the rider off the moped; I'll probably be up on assault charges.