I do not have a BB so I'm not 100% sure what the reason is. However, I assume it could be due to your BB not suporting one of the special APIs we use (JSR-179 for GPS access and JSR-75 for file system access). According to the specs that I have seen, the 8700 should support JSR-179, but not JSR-75. Try installing the JSR-179 Edition of the NavXS client. This has no file system access, meaning that you cannot store map tiles localyy to save network traffic.
I need to get hold of a 'berry at some point
PS: Jack Bauer is our biggest customer