Actually it's a standard practice. Once the theme comes packaged with the software the credits are removed. WordPress can be used for business purposes and out of the box, the link should not be there, but it is in the code which matters.
Your theme shouldn't break when you upgrade because 1.5 accepts both theme sets. However, if you want to move to the new version (which is lovely - not having to worry about the theme breaking when upgrading) you need to save your stylesheet, sidebar and header graphic. I "think" there are some tutorials on this but if not, if you need help send me an email and I'd be happy to try and help. It's pretty easy once you get the hang of it. :)
Actually with the nightly builds (WP 1.5) the Kubrick theme is the default, and the link to the author has been removed. Scoble didn't remove it, it's simply not there....at least it wasn't in the 1/21/05 build I downloaded the other day.
