I just upgraded to WordPress 2.0 and it went better than I thought. There are probably some errors all around, but I will gradually fix them. The themeswitcher plugin is a bit slow. It doesn’t switch the theme right away, only after reloading the page, but hey, at least it didn’t stop working alltogether. 😛
So yes, please don’t mind errors for now, I’m working on it. 🙂
ETA: Upgraded the themeswitcher plugin, so now it works just as it should.