I assume that you are talking about syncing the contacts. While it might not be the cleanest solution, have you looked at using the "ASCII Importer/Exporter" PIM on the Blackberry Desktop and the Text File import export feature of Thunderbird?
Like I said... not very clean, but it may work as a temporary solution.
Otherwise, have you checked RIM's website at the developer section? There is documentation there about creating applications, example applications and conduit code (if I remember correctly).