After updating from 10.6.8 to Lion 10.7.0, my shared folder (shared over SMB) begin having strange problem with my Windows VM.
When I create a .exe file from my Delphi compiler or create simple test file from the Windows VM, OS X doesn't inherit the permissions anymore and sets them to read/write not read/write/execute.
This also causes problems with Windows, which gives me the error "unable privilege for execute".
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.
