MARKUSM I think you will be fine using Tiger till Apple ships 10.6. Apple always tries to target the previous and the current releases of its operating system for its major consumer applications. Other developers normally try to do the same.
Leopard really changed a lot for developers. ObjC 2.0, Garbage Collection, Core Animation, many new default user interface elements, a set of standard icons are only a few things that make life as a developer a lot easier. Especially as a small developer it's really hard to resists. I understand that by focusing on Leopard I decrease my target audience by quite a bit, but on the other side it helps me to be more productive and focus on improving my "product".
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