I think the plugin is regenerating a new short url every time a post is edited and updated. At first I thought it was just supr that was the problem, but I switched to bit.ly and it seems to be doing it again.
Otherwise, sexybookmarks is pretty f-ing cool.
I have the same problem. An example URL (shortened, it actually continues like this for a long time): http://example.com/2010/03/news-archive/article-name/?preview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=truepreview=true (...)