Most are not. Many people have reported shooting Plus P in non-rated guns without incident. Personally I avoid it because to me they're too nice to abuse. I would do it in a pinch. They won't blow up. I'm not sure what model revision marks the Plus P rating (I think it's "-3"), but I think generally the Plus P guns have the elongated, integrated cylinder stop as opposed to the "button" and careful observation would show a slightly longer cylinder window and cylinder plus some radiusing of the corners of the cylinder window to help prevent cracking under heavier loads. IOW, plus P rated Airweight frames are the so-called "J-magnum" frame dimensions.
It might be worth noting that even when this new frame appeared, Smith & Wesson didn't officially advertise Plus P capability until Taurus launched a big campaign touting all their small frame Model 85s as being rated for Plus P. S&W very quickly followed suit. Probably a coincidence.