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Mac OS 10.4 now translates some OpenType features into its AAT environment but not all of them. Notably contextual features are missing. So any .otf or .ttf with OT enhancement for languages such as Arabic are still unusable on Tiger because contextual features are vital for such languages.
I made some fonts that have both OT and AAT features to be compatible with any environment. Now with Tiger, the presence of OT features in these fonts results in partial translation of those features in AAT and ignoring the pre-existing AAT features of the font - including their contextual AAT features!
If you plan to add AAT to an OT font, beware of this problem. It will be useless.
All Apple has to do is to add a command in its text engine that 'if AAT detected in the font, ignore OT'. I know it's easier said than done but this shouldn't be too complicated.