Some others have reported issues when accessing SWAT on a Mac without the Dev Tools installed. One thing worth noting is that this article is aimed at Jaguar users. Panther made a lot of this article deprecated. Today, I tried to use SWAT for the first time on Panther and am encountering the same issue as you. I can't seem to access SWAT, regardless of the authentication credentials I provide. One possible solution is to add a line to your /etc/xinetd.d/swat file which contains:

This will make SWAT run in a mode where it will allow anyone to modify the contents of the smb.conf file (at least, according to swat's man page). It's important to note that if you elect to setup SWAT in this manner, you should be extra sure that xinetd is only allowing connections to the SWAT daemon from localhost. In addition, other users who have console access to the Mac will be able to access SWAT locally.