The propagation characteristics of VHF are substantially better than 900MHz. I've used voice radios in the VHF band in urban Minneapolis, and have been able to easily surpass a mile of range. I'm surprised they didn't choose a lower frequency still.
In addition, using something like DSSS, a low data rate, and retransmission ability will probably help a lot for this application.
Agreed. Substantially better. Our first goTenna prototypes worked at 900 MHz band and we could barely get 0.5 km (NOT MILES) line of sight. With the goTennas that are now out for pre-order, we've gotten anywhere from 0.5 miles to 3.5 miles in NYC (where we're based), to give you an idea -- and there's little that is as non-ideal for RF propagation than a metropolitan city.
In addition, using something like DSSS, a low data rate, and retransmission ability will probably help a lot for this application.