The newest version of the OpenType font format specification, 1.6, has now been published on the Microsoft website:
http://www.microsoft.com/typography/otspec/

Of particular interest, I think, will be the new mechanism for naming of Stylistic Sets
http://www.microsoft.com/typography/otspec/features_pt.htm#ssxx

and the new Character Variant features
http://www.microsoft.com/typography/otspec/features_ae.htm#cv01-cv99

For other additions and revisions, see the change log
http://www.microsoft.com/typography/otspec/changes.htm