OS X Lion SMB shared folder lost execute permission
Samba was dropped in 10.7 in favour of new custom Windows/SMB code - the functionality might still look the same, but the software that enables it is very different.  I have seen various issues related to it, and this could be one of them.  You could always stop sharing via preferences, and install the "real" Samba sharing software and see if that helps - it's not the most elegant solution, but it may prove a point while you wait for a fix.