Or at least I think the change was in Domino 7... I never noticed it before since we have been using a custom DSAPI authentication module to work around the shortcoming in previous Domino releases.
The change is that the @Username function now returns the distinguished name of the authenticated user separated with slashes (/) even though it is a LDAP name and hence really separated by commas. This is über nice since it means we can scrap the custom DSAPI module and move the application from a Domino 5.x server to a Domino 7 server (we are having big issues deploying the DSAPI filter on Domino 7).
(For the record - my custom DSAPI filter does more than convert LDAP names separated by commas to names separated by slashes. It also handles dynamic LDAP groups composed on the groupOfUrls objectClass. If you need something like that let me know.)