1: Free with a new Mac after the release. (I know... going to hell for that one.)
2: As early as March 27th (the 6 year aniversary of the release of OS X), most likely June 11th (the first day of WWDC).
3: We have already been seeing a new UI popping up in the current Leopard builds, however far from as drastic of a change as people seem to be hoping for. However in 9a343, there was the inclusion of a new "Resource Folder" format, in Application Resource files were sorted into <Program Name>/Resources/Aqua_UI/<Files>. This was gone from 9a377a, however current Xcode projects can be made with the new Resource format and work perfectly fine under Leopard.
Apple has said VERY often that Developers should not store graphics related to the UI in their applications and instead rely on linking to the System Resources for controlling the UI. This could just be to make Resolution Independance easier for all applications, but it could also pave the way for Apple to change their UI and have all other Third-Party apps match the style overnight. All speculation though.
4: Absolutely
5: Leopard runs on G3s and up, however to do so you need to install in Target Disk Mode from another Mac to bypass the Machine Check or use a modified install disc. It runs just fine on the G3, however performance has not been that great. But in the end when you are dealing with an absolute maximum of 900 MHz and most sitting in the Sub-600 MHz range, you can not expect performance to be that great. If you have a recent G3 like an iBook from 2003, then you should be fine. Memory may be a little lacking unless you have upped from 256MB but it will run.
In the end, it is not officially supported. Apple says that it will not run on the G3, however I have it running on an iBook G3 with an 800MHz processor and 512MB of memory right now with no known issues. G4s, G5s, CDs, C2Ds & Xeons are supported, everything else looks like it can be made to work just fine though.
