Verizon will never "ditch blackberry." Verizon already has a huge customer base who use blackberries, and it would be a moronic business decision to abandon them.
However, it's possible that in a year or two Verizon may also carry the iPhone in addition to its other phones. That's what AT&T does, after all. We'd have to wait until the AT&D exclusivity has expired, and until the iPhone and Verizon both upgrade to compatible 4g standards.