Not quite. IDLE_N is actually used to control the linearity of the receiver depending if the transmitter is on or off. If the transmitter is off, then the receiver will not suffer from cross modulation (single tone desense). So you can run lower current into the receiver and save battery life. If the transmitter is on, then the receiver will suffer from cross modulation and higher linearity is needed, hence more cruent is fed into the receiver to improve the linearity.
PA_ON is a high speed control line that turns the transmitter stages on and off. If the transmitter is running less than full rate, this line pulses the transmitter on/off for the 1/2, 1/4 or 1/8th duty cycle.
When your handset is just sitting idle, VCC_TX is turned off. When you place a call, VCC_TX is turned on, then PA_ON activates all the transmitter stages. Then the IQ datastream starts from the baseband through the transmttter chain.