This is an absolutely awesome feature. IntelliSense / autocompletion for JavaScript is a difficult problem (because of the dynamic nature of JavaScript) which the Visual Studio team is brilliantly solving. They were able to build an elegant solution to the problem that works reasonably well on plain JavaScript, but really shines when used with ASP.NET Ajax. The type information that we added to our debug scripts through doc comments really helps in making the whole experience seamless.
Jeff King wrote a great blog post that shows what's already available in the Orcas CTP:
http://blogs.msdn.com/webdevtools/archive/2007/03/02/jscript-intellisense-in-orcas.aspx
The CTP itself can be found here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=b533619a-0008-4dd6-9ed1-47d482683c78&DisplayLang=en
