There should not be any difference based on SQL Server version or Access version. This technique is dependant only on SQL Server, and we have similar tables that work just fine on both versions you mention.

On permissions, we normally give each user dbowner on the database so that we don't have to set permissions on all of the objects - as in general our users should be able to update all the tables. (We do restrict users by presenting the data in forms and do not give them access to the tables.)

Just a hunch, but by chance are they using an Access form to add records that has a different default value set for that field? That seems to me to be the most logical explanation for the behavior you are seeing. To test that you could try adding a record to the table directly without using an Access form. Another more remote possibility is that there is an issue with the UserName in SQL Server - is this Integrated Security or SQL Server Security? Also it would help to know if the field is truly a Null, a series of blank characters, or an empty text string.