If UMD movies can sell well, I can totally see Nintendogs selling a few copies here and there, but it'll never outsell Grand Theft Auto, not in this reality.
as free-roaming as GTA3, ie - loading between cities but totally free. Missions and cut-scenes will be shorter so you can pop off a mission or two on a short car ride, but there is still total freedom. No planes/helicopters, but motorcycles will be there (bikes in LC!) and there will be new cars as well, all adjusted appropriately for the time period.
All I have to say is "snow in a GTA game." Just the fact that they are doing something on a portable GTA game that they have never done before is HUGE!!!
It's interesting that they teased it in GTA:San Andreas too (when CJ takes that plane ride). They've hinted at stuff in future games before (or, at least, there were details that people picked up in hindsight), but this is pretty direct.
And the fact that this is PSP only should help its sales that much more. Their should be at least 3 million PSPs out in NA and Europe by time GTA is released.