Nobody's claiming that "all" Mac logic boards are defective. There is, however, definitive evidence of a disturbingly high failure rate if one clicks on the links in previous posts in this thread, and in Apple's extension on original logic boards on some of the early G5s. And no, these failures are not triggered by coffee, paper clips, power surges, or cats. There seems to be something cult-like about the denial of the seriousness of this problem on the part of dedicated Mac fans. Really, it's nothing you need to get so defensive about. Closing one's mind with religeous devotion to a brand is one of the worst mistakes a tech consumer can make.
In another forum, one repair professional offered the opinion that, while component failures occur at varying rates on specific models for all brands, brands that use a lot of proprietary component configurations (the example was Dell) leave fewer options for replacing defective components and are more likely to have recurring problems after repair. For obvious reasons this would also apply to Mac, supported by the number of people having recurring problems on one component.
FYI - I migrated to Mac as a result of the Windows 95/98 stability and security debacle. However, it's a myth that Mac OS is inherently more secure than recent versions of Windows. Mac OS also has its security flaws, but they have not been targeted by as much malware and Macs are less likely to be used in networked environments, which accentuate security flaws. Also, having seen the vast improvement of Windows XP over earlier versions in terms of stability, I'm not convinced that there's still an advantage to Mac OS at this point. Looks like time to migrate back and get away from the disadvantages of a brand that has more proprietary components than any other.