Not all of the Safari changes can be backported to Jaguar, because they rely on new graphics APIs in Panther. The most obvious of these is text-shadow support.
However, Apple has to release the source code to WebCore at some point in the near future, thanks to KHTML's license. Once that is done, most of the other changes could be backported and then dropped straight into Safari. This wouldn't do much for the UI, but would fix some rendering bugs and add support for a few more standards.