(You should never type postings during your working time when you're on the phone at the same time
)10.5 will be released 2007 I think, since Jobs said that the last OSX versions where released to close to another.
Apple worked on cross-plattform versions of OSX for about 5 years as a safety net, so there won't be an emulated version of the os, but maybe for some of the apps. Some of the apps can be cross-compile with minor changes. And apple has a year time to optimize their main software apps for P4.