Developer builds of the latest Mac OS X build 10.6.2 were unable to run on netbooks built on the Intel Atom processor. This was significant as many of those who have hacked Apple's operating system to run on a PC were doing so on netbooks.
While the hackintosh community has been mostly ignored by Apple thus far (unless you're a commercial business selling hackintoshes), this marks a proactive step for the company in stomping out unauthorized usage of its software.
Those who have taken the plunge to 10.6.2 have discovered that their systems are now unable to boot, as it just puts itself in an infinite loop of restarts.
Oddly enough, those with hackintosh computers built using other Intel processors that aren't the Atom are not affected. This means that Psystar computers will accept the update without such an issue.