There have been rumors floating around in the past that the BlackBerry facebook app would aquire the chat feature. This sounds promising if the rumor becomes true as we have heard from users of the facebook v1.8 beta that the app is much improved from the current official release.
Although, reports on v1.8 say there isn't a chat feature, but that the overall experience is much faster. Now, it seems that facebook has enabled it on their end to allow the facebook Chat on a BlackBerry. There have been third-party apps that allow you to do facebook Chat, however, this appears to be for the official facebook app that is developed by RIM.
We're hoping that we will see this feature in v1.8 even though reports suggest otherwise. Either way, it is a good sign that facebook has taken the initiative to get their backend setup in preparation.