In a follow-up post, Atwood points out the font design and rendering is incredibly hard. It's not surprising the Apple would choose to be "true to the design" and Microsoft would "do what works." Who's right? Both Atwood and Dave Shea make a strong argument that Microsoft's approach is more pragmatic and results in text that's easier to read. I'm not sure this difference will result in fewer users for Safari. Some people like the way the Safari rendered text looks. I suspect in any event that people who choose to use Safari on Windows rather than IE or Firefox probably are making that decision for reasons that will trump font rendering and any resulting bluriness.
Apple generally believes that the goal of the algorithm should be to preserve the design of the typeface as much as possible, even at the cost of a little bit of blurriness.
Microsoft generally believes that the shape of each letter should be hammered into pixel boundaries to prevent blur and improve readability, even at the cost of not being true to the typeface.
Via Robert Scoble, I ran into an interesting discussion of font rendering in Safari running on Windows. Jeff Atwood has some side-by-side comparisons of Safari rendered text and IE rendered text showing the Safari's font rendering is clearly blurred (sorry, I couldn't resist). Joel Spolsky explains that the difference can be explained by different philosophies about how to "coax sharper-looking fonts out of typical low resolution screens." Joel puts it this way: