EV is already programmed into the HV computer. To activate EV, you are momentarily grounding an otherwise unused pin on the ECU, as the other international markets have their EV button do. They also have an indicator on the dash showing that they are in EV. We don't have that light available to tap into, though Attila has found that the computers do provide that data on their network.
EV is allowed if the car has not started the engine for its warmup phase, you are under 35 MPH, and you have at least 3 bars on your battery. Cruise control is disabled while actively in EV mode.
It will kick you out of EV if you accelerate too hard, you have drained your battery down to 2 bars, or you have reached or exceeded 35 MPH.
It was speculated that EV was not provided in N.A. because of emissions certifications.
Toyota implies that warranty would be void, but we aren't sure if they are referring to a pluggable mod with added battery capacity, or if they are including simply adding a switch to activate a feature already incorporated into the car. I really doubt Toyota could win a case on this if they did try to deny warranty solely on having an EV switch. They definitly would lose if the otherwise warrantied item is not affected by the EV, such as engine problems, or even a motor/generator problem. They MIGHT have a case with the battery, but even then that would be reaching.
Using the EV might affect your battery life if you routinely use it and drain the battery down to minimum, then charge the battery with ICE to full (or as full as it would normally get, which is 6 bars). This is where Toyota might have a case in denying warranty. You could achieve the same 'abuse' by frequently draining the battery by idling the car in READY mode, running the A/C, and then driving, thus charging the battery to 'full'. This is where Toyota would lose their justification to deny warranty on the battery because of EV.