when i upgraded to fedora 13 i ran into an issue of changing my ISP service at the same time. Going from Cable to DSL i ran into problems with Mozilla, Pidgin, and Evolution not connecting i searched here and google for 5 days trying solutions and tips they had posted but in the end i noticed that the DNS set up by the DSL was using my modem/gateway address so I contacted the provider and got the dns ip for the primary and secondary dns servers and once i entered them everything workd.
Hope this helps