Will version 3.5 support encrypted DOCX or XLSX files generated by MS Office 2007/2010? In the current available version (RC3?), these files are not recognized by Calc.
Short answer: no - once a release is out, new features are not added to it.
There is a bug reported about this for LibreOffice 3.4 (#fd35422). Please subscribe to the bug (unless you’re the reporter). You can also bring attention to it via forums, social networks,etc., however reverse-engineering or even implementing features from other formats (even from documented sources) may take some time as other priority tasks may take the existing resources already.
The latest update for fdo#35422, on 2012-12-21 by dev Kohei Yoshida:
“I’m turning this into an enhancement request. This is not a bug; it’s rather a missing functionality. Excel 2010 uses different encryption method than that of Excel 2007, and we’ll need to write a new handler code for this.”
fdo#35422 – Status: RESOLVED FIXED