Anyway, I wanted it integrated into Aptana's Studio, so I took the client libraries and bundled them into a JAR file as an Aptana (Eclipse) plugin. You just download this one JAR and save it in the Aptana\Plugins folder and when you make a new Web Project, Microsoft Ajax 3.5 will appear in the list.
...as well as integrated with the online Ajax Documentation
I think that JS components like MS-Ajax should be easily accessible in other IDEs that aren't Visual Studio, like Aptana.
The one big feature that I didn't have time to do was full Intellisense (Aptana calls it "Code Assist") as it'll require a custom ScriptDoc XML file to describe each method within the libraries. This means that you'll not get the extra documentation tooltip, but I'm working in it.
Bertrand has a post explaining what our format for JS metadata looks like. The Open Ajax Alliance has an IDE Charter that describes an interoperable way for IDEs to interact with libraries, publish metadata,etc. Microsoft joined the Alliance last year and passed the Open Ajax Interop Test last September. Bertrand has a whole category dedicated to Open Ajax. When these specs gets better figured out, we'll make sure this gets supported. I may write a tool in the short term to convert between our format into Aptana's if folks ask. Eventually, however, everyone will use the standard (after it comes out, you know how standard take time).
I've been talking with Aptana to get this support built in and possibly auto-downloadable, but for now, if you're running Aptana and you'd like MS-Ajax support, save this JAR (com.ajax.ms.3.5.21022.8.jar) into Aptana's plugin directory and restart the IDE.
Later, I'll show you how to do the inverse with Visual Studio, integrating Prototype and JQuery.