The question of the warranty has been discussed. But since I'm not a lawyer, I cannot give you a definitive answer. I'd say that for the installation to affect your warranty, there'd have to be reason to believe that your installation caused the problem for which you are seeking warranty work. I don't believe they could cancel your warranty across the board just because you had made a modification. But that's just my opinion. Of course, if you flub the installation itself, the responsibility is yours. I don't know if an improper installation can fry the computer, but if it does, you pay for the new computer, and I bet that's expensive, and a long wait for the replacement.
The swings in SOC are still controlled by the computer, which will cancel EV mode the instant you drop to 2 bars, so SOC swings are not significantly different than in normal operation. Also note that the Japanese and European versions have the EV button as a factory standard, and that the DIY installation merely uses the pin already in the car, which signals the program already in the computer. You are not overriding anything Toyota has built into this car.
Folks over on Prius Chat have speculated that the car probably does store information that might tell a service tech that the EV switch has been used. I think there's more solid technical knowledge over on the Yahoo groups, if you can deal with the Yahoo user-group format. I cannot.
Finally, let me repeat that the real utility of the EV switch is minimal: it's intended for when you are going to move the car a short distance only, or want to keep the ICE silent for a few blocks of neighborhood driving. It is mainly a techno-geek kind of thing. I like being able to control what the car does, and I'm willing to take what I consider a small risk over the warranty. Your results may vary.