We’re a bit behind on this so I apologise but better late than never. The Pika Club has unearthed some footage from Pokemon Sunday showing a short clip of Heart Gold/Soul Silver in action.
Significantly the footage shows the trainer wandering around outside Slowpoke Well with Totodile following him. This would appear to suggest that the Pokemon follow you like in Pokemon Yellow and are not limited to any particular areas such as New Bark Town.