When Apple released 10.4, they committed to making no more API changes that would break existing software. So assuming they stick to that promise I shouldn't have to do anything for Leopard. Because I plan to maintain 10.4 compatability, I am also not rushing to add anything that relies on 10.5. I'm certainly excited to check out the new APIs, however. If it makes sense to add features that will only work for 10.5 users, I'll surely consider it.