I think that they shortened the testing period. Sony is far more experienced with electronics than Microsoft and is very much aware of the repurcussions it would have if the unit is released before extensive testing.
Although Sony had a similair problem with the PSTwo's psu it immediately recalled the faulty units and offered a free replacement for the defective ones.
I also read that there are a lot of people already on their third 360 after sending it back to Microsoft. It seems to me that they do not correct the problem but simply send a new 360 with the same design flaws.
Microsoft also probably wanted to be the first on the market with a nextgen console. Perhaps they believed that they could just simply fix the problems via a software patch. Microsoft is very aware of this overheating problem which is proven by the fact that they released a dashboard update which enables the fans to run full-speed. I also believe that the relative short period of warranty is linked to the quality of the product unfortunately.
The PS3 will be able to overheat as would any other device that is capable of producing heat. If you put your mind to it you can overheat anything. But after inspecting my trusty old PS2's I believe Sony will deliver a fine console. The PS2 has one large heatsink spanning all the essential components CPU,GPU,APU, Memory, Controllers, etc and a small fan with a straight airflow from the front, over the heatsink and through the back of the console. The 360 is trying to suck the air out of the back of the console and through two massive heatsinks with almost no room to breathe. Why for Pete's sake would you suck air from an almost standard pc heatsink when blowing air through it gives much better results?
All in all I think Microsoft is letting down their fans(...lol) by their stubborness and their poor customer service. I believe that this may well be the most ill-manufactured console to date.