I've been running Leopard...
for just over two months, and I've not really seen a difference with regards to my iPhones. Though I've been able to download some awesome wallpaper, I've not been able to duplicate Leopard's new "screensavers" on my iPhones; though they look really cool on my MacBooks and Mac Pro. I noticed some printing issues, but those were immediately remedied. As others have said, "syncing" issues with iCal seemed to be commonplace, and currently are non-factors. I'm definitely getting ready for additional iPhone & Leopard workshops, plus the "iPhone Supersession" at Macworld, San Francisco, CA, is coming up in a few weeks! 😎