I found a nice article on Twitter outage here -> http://www.25hoursaday.com/weblog/2008/05/23/SomeThoughtsOnTwittersAvailabilityProblems.aspx
An interesting though. However there were some mistakes in his assumptions, and he has clarified that here -> http://www.25hoursaday.com/weblog/2008/05/26/SomeThoughtsOnSingleInstanceStorageAndTwitter.aspx
Stuff makes an interesting read on twitter and its architecture, and general issues. Interestingly I found this site from http://twittervision.com 
