The speaker-port cracks aren't the result of rough handling--I've babied my TP since day one, and still got one crack within the first month.
Being trained as a mechanical engineer and having some experience with product design and manufacturing processes, here's my guess as to what's going on:
- The TP's case is molded plastic; it's likely that HP used a molding process that introduces stress patterns into the plastic as it cools (this is very common, BTW). These stress patterns are much more common in curved areas, with the incidence increasing as the radius of the curve decreases.
- The speaker ports are cut into the case after molding, rather than being part of the molded design (I'm inferring this from the sharp edge of the ports--molded holes would have a slightly rounded edge).
- When a cut edge in a molded plastic product intersects a stress pattern in the plastic in just the right way, a weak point forms, leading to the possibility (or even the likelihood) of a crack's forming at that point. I'm pretty sure this is what's happening with the TP cases that have developed cracks.
So why don't all TP cases crack? The distribution of stress patterns in the cooling plastic is random--whether a case develops a crack at a speaker port depends on that specific case's stress pattern. Essentially, it's the luck of the draw.
The bad design decision on HP's part was to specify that the sound ports be cut into a small-radius curved area of a molded plastic case. They should have either specified a plastic molding process that doesn't introduce the stress patterns or planned on molding in the speaker ports, rather than cutting them out. I'd guess that the former approach might have significantly increased the cost of production, however; I'm not familiar enough with complex molding to even guess what cost impact the latter approach would have had.
In any case, we're stuck with a poor product design decision. It's one that surprises me more than a little bit, given HP's experience with making plastic cases for small calculators and test equipment.