Most themes that are compatible with WordPress 2.9.x should be compatible with WordPress 3.0, so I would hope that the Default theme would still work. You’ll just miss out on all of the new and nifty 3.0 theme features.
I have not tried 3.0 yet, but I’m assuming that you shouldn’t have to do anything, since all of the customizations are saved in the theme files. Just in case, make sure that you backup your files and database before upgrading.
The Default theme with 3.0 is called Twenty Ten, and it’s a completely different theme, so you probably won’t be able to copy things over, but you should be able to implement your customizations from Kubrick. It will be like starting over, but this time you have a plan and know exactly what you want.