I would like to add a little to this : Super thin tubes are more likely to break than thick tubes, almost regardless of the quality of the steel. In other words, a frame made with 0.55mm butts (the tubes are out there), even if it's Jetson-age tubing with wonderful metallurgical scores, is closer to death than is a frame made with Hi-Ten steel that's 1.2mm at the butts. (My opinion, just from what I've seen in the past 20 years.)
This is generally true, but not for the reasons most might think. If we make a comparison between a 1 1/8'' .9.6.9 downtube like SL or the like and a modern 1.25'' .7.4.7 True Temper Platinum tube we will find that they have nearly identical stiffness or (moment of inertia) and therefore have a similar stress level. Theoretically they should have similar durability. The upside is that the Platinum tube weighs 70 grams less than the older, thicker one. The downside, in my opinion is that the Platinum tube has a larger ratio of thickness to diameter and therefore is more prone to buckling failure if it is dented. So I would say that some modern tubing is just as durable it just that you cant ride around with a big ol dent in it like you might have been able to in the past. I am with Grant on the super thin .5.38.5 ultrafoco tubes (is that even butted?). I don't see them having the same reliability that standard tubing has.
Dave Bohm
Bohemian Bicycles
Tucson Arizona