The biggest obstacle for a device-based GPS software right now is the current 2gig limit on the MicroSD support. Unlike other devices where the storage card is readily accessible, the BB is a pain. Once we move to larger cards, then I'd see a market for flat fee, downloadable data.
The price, other than data fee, is not different. I have used both 'on device' and subscription units, and if you want to keep current maps, both will run you about $100/year.
I woul actually see a great markt for a choice between a 'buy once' or a subscription with maps on SD card. So that you would have choice of paying for the maps whenever you felt the need for new maps, or getting the 2GB once, and then being able to download updates as they become available, either to your desktop or OTA.
Then the BB would really be the ultimate device.