I just got around to looking at the Spring 2002 issue of the Journal of Computer Resource Management (unfortunately, there's no online version, but its published by the Computer Measurement Group).  There's a nice article by Chris Overton called "On the Threory and Practice of Internet SLA's."  Chris works for Keynote Systems and does a lot of this kind of analysis.  This is probably one of the most comprehensive analyses on service level agreements that you'll find anywhere.  Worth a trip to the library if that's what you have to do to read it.  Baseline magazine had a related article (that references Chris's article) in their March issue.   

I just received a note from Chris and he's made the article available at Keynote for download.  Eventually (sometime next week?) there will be a spreadsheet tool that accompanies the article.