During Beta1 and Beta2 pre-releases of IE8, we’ve blogged about the performance optimizations done in the Script engine and the addition of new language features such as native JSON support. We also provided details about the JScript Debugger and the JScript Profiler –that shipped as part IE8 Developer Tools
…
The JScript engine now exposes new language features only when the layout (document) mode is set to “IE8 Standards Mode.” Versioned JScript language features available in this release include native JSON support and accessor methods for enabling Document Object Model prototypes.
Source : IEBlog