So, I thought I'd throw this out there as I don't recall ever seeing a concrete conclusion on the topic.
I know it's been stated before that some people have experienced situations in which the navigation buttons on the ZX2 stop responding intermittently. Having used the ZX2 for 1.5 years, I had never experienced any such issue. I, and I think many others, chalked it up to perhaps a manufacturing defect in a certain range of units.
In any event, recently, with all the talk about the 1Z and 1A, and the debate over their lack of Wifi, I began thinking about my own usage patterns on the ZX2. Frankly, I had disabled all Google Apps and have left Wifi turned off for, probably more than a year. But, if only to prove a point to myself that Wifi is still a useful addition (if only for music discovery), I re-enabled all the Google Apps, turned on Wifi and streamed an hour-or-so of Pandora.
Afterwards, I turned off wifi, but left the Google Apps running. I wanted to see if perhaps my runtime between charges would be significantly impacted.
Well, after about a day, I noticed my navigation buttons stopped responding until I unlocked the screen. Odd. Oh well. Back to listening. And the next day, it happened again. And the next.
Okay...Let me reboot. Perhaps Pandora was doing something naughty in the background, trying to steal focus on the navigation buttons or something.
So, a few hours after a fresh reboot, and the navigation buttons stopped working again.
Alright. Time to go back and turn off all the Google Apps, and reboot.
Since doing that, I have not yet had any more instances of the navigation buttons becoming unresponsive.
Not necessarily proof of anything—as there might be multiple ways to make the issue manifest. But, from my perspective, I suspect one (or more) Google-related apps are behaving badly or are otherwise interfering with the navigation buttons after a period of time. So if you're encountering that problem, you might want to try disabling them to see if the issue goes away.