After updating from 10.5.8 to 10.6.1 I was not able to use screen sharing any more to connect to a machine running 10.5.8 If I click on "Share Sreen..." in finder, by default my short name is used (e.g. jwhite) Using the correct password I was faced with the message: (Authentication failed to "...".) Please verify you have entered the correct name and password. This worked fine in Leopard to Leopard. Snow Leopard to Leopard did not work. Trying a lot of things I managed to use screen sharing without a problem by using the full name (e.g. John White instead of jwhite) Hope this helps someone facing the same problem.