Files created by Windows on Lion SMB shares have 600 permissions
I think this could be a bug. I'm seeing the same behaviour in Mountain Lion too.

I've ensured that the file sharing settings are correct. Go to the folder in Finder and verify that the user has correct permissions, including creating files locally, which result in the correct (644) permissions. Go to System Preferences -> Sharing and select the share, and verify the user is in the list with read&amp;write permissions.

Using the command line tool:

$ dscl localhost -list /Local/Default/SharePoints


to list shares, and:

$ dscl localhost -read /Local/Default/SharePoints/&lt;sharename&gt;


I can see:

dsAttrTypeNative:smb_createmask: 644
dsAttrTypeNative:smb_directorymask: 755


Yet files created by windows users end up with 600 permissions and are not readable by others.

While not an answer, hopefully it will give someone a step closer to finding it.