Borders has a new e-book application for the Blackberry, available OTA here. Not a bad little app, with plenty of books (including many free) available. See a number of screenshots below: browsing by category, viewing a book's preview, search functionality, and reading a book you have downloaded...
The app is set up to transfer your purchased e-books to the Kobo E-reader, which Borders also sells... Interestingly enough Kobo has it's own e-reader for the BB and other platforms (iPhone, Android, Palm, etc.), see here.
Borders Launches Free eBook Store with Free Books Upon Download
Borders Launches eBook Store, BlackBerry and Android Apps