I was listening to the Today program on BBC radio, and there was some good commentary. Basically, the Israelis 'red line' is the implementation on more centrifuges (where the Iranians are now). The USA's red-line is the creation of nuclear weapon grade uranium. They both will not let Iran create a nuclear weapon, regardless of what the Russians or Chinese may say.
No-one can see the Iranians stopping their research, so come late summer, the Israelis will do something, with or without the US.