Possible to allow READ & WRITE but not Delete? [Archive] - Mac Forums:

chmod a 'johndoe allow read,write' /path/to/folder
chmod a 'johndoe deny delete' /path/to/folder

The effect of these two commands will be to allow user johndoe to add files and remove files from /path/to/folder, but he will not be able to delete /path/to/folder.

Possibly useful.