There appears to be a big difference beteen the alleged problems with the Nano & the HD5 buttons.
When the HD5 controls crack, the problem appears to be entirely cosmetic.
When the Nano screen shatters, or quickly looks like it's been rubbed with sandpaper, distorting the images, the unit is no longer fit for normal use.
Of course, the problems are only alleged at the moment, may be limited to one or two small batches, or may soon be recified by Apple, with all faulty units replaced. Time will tell.
However, this does once again flag up the problem of having something so small.
All of the problems mentioned appear to have happened because people have been carrying the Nano in their jeans pocket. When a unit so small is created, people are bound to squeeze it into their jeans. The problem is that these pockets will be tighter on the unit than if it were sitting loosely in a person's jacket pocket, or hand held.
Unless metals and plastics are considerably strengthened, it will never be advisable to put a unit with a screen in your jeans pocket. Subseqently, creating a unit who's sole advantage is that it will fit in your jeans pocket becomes less desirable.