I would imagine so, unless Apple also discontinued the XP version, which I don't see them doing. It would be silly if Safari 5 ran on XP but not Leopard.
Well, it would be silly if Steve Jobs worked at one company that sold Blu-ray discs and another that made expensive computers that didn't support their playback... something being silly doesn't always rule out Apple thinking it's a good idea.
I don't think Safari will be dropped for PPC machines yet, and hope not. If they do, I'll switch to Firefox as my main browser, I could happily do that if necessary.
Kudos to the post that mentioned how security should be the priority in a browser btw, I agree with that. I'd be happy for Safari to add features to newer versions of the OS as long as the older version is updated for security for a while yet. I'd also like the option to turn off the Top Sites feature altogether in the preferences instead of having to lock the folder where it wants to cache tons of screenshots.
I find clicktoflash makes everything nicer, and for even more speed set a keyboard shortcut for turning off Javascript when you don't need it (I use command-J).
Also, I think some people really need to reassess what 'dead' means. Say 'dying' if you like, but calling things 'dead' that are still in use by lots of people really is silly. You wouldn't call a person 'dead' just because they retired or became a grandparent. Wait until they actually require a funeral before dancing on their grave ok?!