The old rule of 60w maximum was ostensibly about the fixture's ability to stand the heat generated by incandescent lamps. They do their thing by getting literally white hot, after all.
Really, the rule has everything to do with a company wanting to shed as much liability as possible. If you go and burn your place down because you used a 100w bulb in a fixture rated for only 60w, then it's all your fault, right? Right. Because you ignored a safety warning.
This new rule is because somebody didn't understand the rationale behind the old one. Heat from the incandescent lamp was the problem there and always was. CFLs get warm but nowhere near as hot as an incandescent lamp. You can touch a 100w-equivalent CFL while it's at full brightness and has been for a while; it'll be uncomfortably warm but you won't be burned like you would on even a 40w incandescent. But all the ratings on a CFL give an incandescent wattage equivalency, and that's what the rule makers are looking at. They aren't thinking, that's all.
You could safely operate a 60w CFL in a 60w-rated incandescent fixture - assuming you could get the lamp to fit. It would give off light comparable to a 250-300w incandescent lamp. It would be enormous compared to the incandescent lamp. But the dissipated energy as heat would be the same, and so the fixture would be able to handle it.