If you are beginning a new development effort, chances are pretty strong that you want to hit the ground running with Leopard. There is no existing customer base to annoy with the switch, and you can get access to the latest and greatest technology right from the beginning.
A new development effort tends to target a different audience than an existing application that has a history in the marketplace. In the case of a new application, you usually want the early adopters and risk takers to take a look at your application.
These risk takers are the same people who will be lining up to purchase Leopard as soon as it hits the shelves. Therefore, your release of a new application will grab their attention by the very fact that it is available only for Leopard.
If you are in this position, you should try to take advantage of the advances made in Leopard at every opportunity. Make sure you use Core Animation (but don't abuse it), Core Data (where appropriate), and so on.
These technologies were added to or enhanced in Leopard specifically to make applications look and work better. Use them to the fullest extent possible.
This will accomplish two goals: First, it will make your application more appealing for purchase. Second, it will make it clear why your application is running on Leopard and not Tiger. Once you release your new application, you will probably hear some complaints about why it is not Tiger-, Panther-, Jaguar-, or Cheetah-compliant.
Pointing to the Leopard-only interface enhancements will help make that decision clear.