We have received reports of this behavior for some Leopard users, however it has only affected a very small number of users.
This TechNote describes the problem a bit:
Dreamweaver CS3 could not connect to FTP site after Leopard (Mac OS X v10.5) upgrade
If upgrading to 10.5.2 does not resolve the issue, then the other workaround is to use the IP address of the server and/or try clearing the DNS cache by using the following command in Terminal: sudo dscacheutil -flushcache
Ryan M
Adobe Dreamweaver