While this is not directly responsive to the question above, here is the fix for Windows machines.
1. Close all runnning instances of Acrobat
2. Open regedit
3. Navigate in regedit to HKCU\Software\Adobe\Adobe Acrobat\10.0\Annots\cAnnots
4. Search for "opacity"
5. You should find key with the name "dopacity" underneath something like "cFreeText_003aFreeTextTypewriter"
6. Change the "dopacity" value to "100.000000" (that's six zeros after the decimal point)
7. Restart Acrobat and it should work.