This question now gets a little trickier. If you want to make substantial modifications to an existing application, the answer is not as black and white.
It all comes down to the scope of the changes. If you are adding substantial new features to your application, and these features would be further enhanced by the new technologies available in Leopard, the answer might very well be to go for it!
However, if the enhancements you have scheduled are still doable in Tiger, you would still probably be better off waiting before making the switch to Leopard-only releases.
As mentioned above, not all of your customer base will switch to Leopard right away, but those existing customers will be early adopters of your next release. This is especially true if it will be a free upgrade for them.
If you keep the entry barrier very low (that is, a free upgrade and no OS update required), these users will happily upgrade.
If the release you are working on will include substantial improvements, and those improvements would be furthered by moving to Leopard, it is now probably the time to make the switch.
However, as mentioned above, keep in mind that the switch to Leopard will force some people to either stay with Tiger or purchase a new machine. That should be kept in mind when deciding on this course of action.