This is a really great plugin. It just works out of the box!
One suggestion: include the Highslide checkbox in the image editing dialog as well. You know, the one that comes up when you edit an image that's already been placed. This way, we have the option of turning on your Highslide option on older images.
Also, is there a way to navigate between images on the same page, kind of like the old Lightbox worked? Or is that not part of the Highslide code?
I'm very impressed! Nice work! Btw, I'm using WP 2.7 and this is working fine. May want to update your status.