"... So how do you define success? Is Arc successful if you open a web page and then 30 minutes later try to submit a form and get "unknown link" or something to that effect. ..."
I observed this in 2007FEB22 , you can see it here ~ http://www.flickr.com/photos/bootload/398269769/ and the classic response ... "... wouldn't have been a timeout; we probably restarted the server ..."
Though I wouldn't call a broken piece of a webapp (hackernews) a failure of Arc.