It's always a boon when someone gathers the greatest hits of some massive drop of new and complicated developer resources into one simple page that serves as a roadmap to deeper understanding. If you're ho-hum about porting to Leopard, you and your users are missing out.
I wish I had a knack for what Matt Gemmell does. His list of top reasons for developers to love Leopard will make Windows and Linux developers green with envy, and convince Mac developers to modernize and simplify their code. Matt's list is simple, concise, enlightening and enlivening, and I recommend it even for non-developers.
Along the same lines, is it Kosher to set Leopard as a requirement for your custom app? Like all responsible writers and speakers, I leave that to you. Extensions to Quick Look are emerging at a more rapid pace than Dashboard widgets for now.
I think literature like Matt's and WWDC '07 session materials make "OS X 10.5 or later" badging an awfully appealing prospect, even if it is politically incorrect by historical practice. At the very least, you might consider forking your projects and let versions addressing OS X prior to 10.5 slide into maintenance. There is so much developer goodness in Leopard that can't be back-ported to Tiger.