I know this comes in a bit late from me, but nevertheless.
The Code Focus Magazine – 2008 – Vol. 5 – Issue 3 was focused around IE8. If you did not get a chance to look at it as yet, and are interested – you could download a PDF copy of the same from here.
There are sections where-in you could find more details around the JScript Debugger, JScript Profiler and the Native JSON Support apart from others. These features are available with IE8 Beta2.