The built in conversations chat is just a multiprotocol client. It doesn't use any nokia servers at all - only the Ovi chat obviously does.

None of your FB/GTalk/Jabber/Skype/etc chat services will stop working until the servers you're using change the protocol, and even then aside from skype, the plugins are all open source (using pidgin's libs, no less), so you have nothing to worry about.

I much prefer the integrated clients to pidgin, but maybe that's just me.