Today, on the OpenType developer discussion list, Microsoft made this announcement:
A new white paper has been posted to the Microsoft Typography website. The paper was written by Greg Hitchcock to help font designers understand the Microsoft implementation of the TrueType interpreter.
Here's Greg's official summary:
With the implementation of ClearType® in Windows XP and later versions, adjustments were needed for the interpretation of some TrueType® instructions in order to allow existing fonts to work with no updates. This white paper is targeted for font designers and describes the instruction changes and how fonts can continue to have their instructions behave as described in the current OpenType® specification.
Note that this white paper is not part of the OpenType specification, and will not be submitted for inclusion in the ISO Open Font Format. It is provided solely to explain how Windows handles hinted fonts under ClearType rendering.
The paper can be found here: