Mac OS X 10.6 Snow Leopard introduced a lot of under-the-hood changes and many are not very obvious. One such change is to the authentication requirements for logging in remotely via SSH.
In 10.5 logging in remotely via SSH was a pretty standard affair. In 10.6, however, security has been beefed up a bit to require case-sensitive login credentials. While this requirement has already been imposed on passwords, Snow Leopard now requires a case-sensitive user name as well.
In other words, when logging in via SSH, Snow Leopard differentiates between the username "aron" and "Aron." This threw me for a loop for quite some time and is another one of the numerous reasons I have held off upgrading my Mac mini to 10.6.