There are several commercial XMPP clients for BlackBerry that you can use to connect to any standards compliant XMPP server.
However, I believe only the client available at
Vayusphere Jabber/XMPP Instant Messaging Client for BlackBerry is able to leverage the BES infastructure and does not require the use of a 3rd party proxy ( which may be a security issue for some users)
BlackBerry provides an enterprise IM proxy but that is only for the IBM, Microsoft and Novell messengers.