hey Dearheart!
I (very anxiously) read this book too (big Ripper freak here), and was disappointed when her theories were "ripped" by many experts...turns out she made some serious errors and leaps of faith in her research (most notably that her suspect--Walter Sickert--was in France during 4 of 5 of the murder nights). I don't think we'll ever really know, but this case, judging by most of the reviews I've read of Cornwell's book, is far from "closed"!
http://www.casebook.org/dissertation...ndsickert.html
http://writ.news.findlaw.com/dean/20021122.html
http://www.suite101.com/lesson.cfm/18593/1951/4