I agree that ClearType loses a lot of its benefits at high DPI and, similarly, when you have giant font sizes.
I covered this in Respecting the Pixel Grid:
I don't understand why Apple is asking us to sacrifice the present at the altar of the future. Can't we have hinting at low resolutions, and accuracy at high resolutions, too? Snapping fonts to a pixel grid may very well be irrelevant when everyone is luxuriating in the glow of their 200 DPI monitors. Until that glorious day arrives, respecting the pixel grid certainly makes text a lot more readable for those of us stuck in the here and now.
These two strategies ...
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. (OSX / Safari default font rendering)
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. (ClearType)
... are in some ways mutually exclusive -- one looks bad at low DPI, and the other looks bad at high DPI.
In a perfect world, we'd dynamically switch between these two strategies depending on what DPI and font size we were using. I am not aware of any operating systems which dynamically switch font rendering strategies at certain DPI levels or font sizes, but they should exist!