I understand the on-the-go mobile aspect of up-to-date data and not relying on the device to have enough memory to download to disk (or flash in this case). BUT, would it be cost-effective from a network infrastructure? How much data is used in polling and repolling for information? Would it not be a little more efficient to load that street or that satellite photo from a fixed location on your device and efficiently use the network for what its intended: real-time location ONTOP of existing maps? You could update your device once a month from your computer and be fine.
I guess I'm saying all of this because I'm reaching for another outlet to rationalize me having a blackberry, paying the monthly cost for unlimited email/data plan, and this would be a nice little extra to feel like I'm paying for in the big picture.
Seems to me this would have already been developed but service providers are smart--they know what makes the most money. Hopefully it will be more open-source in the future but I'm not holding my breath.