I had a great discussion [1] with Bertrand [2] about the feature to have dynamically (Assembly Resources or on-the-fly created JavaScript files) added JavaScripts in IntelliSense available, too. Currently it is only possible to add JavaScript files by hand that are more or less static files, and ASP.NET AJAX does not come with plain JavaScript files, too, but it is working because ScriptManager is doing his job.
Talking about this missing feature I got following answer:
Most other frameworks that you'll want to get IntelliSense on come as plain JavaScript files. With what limited time we had on Orcas, we couldn't include the case of dynamically added files from server code. The reasoning was that this is a less common scenario as most of the times the control is going to be the only user of the client code it includes and the page developer will rarely need to work directly against this form of library from client code.I would personally have loved to see that feature in Orcas but getting the product in the hands of the customers is more important at this point.
So, we will wait for Service Pack 1 for Orcas... but I think most of Web developers can live with that because we didn't had JavaScript IntelliSense until now.
One think more is that Microsoft joined the OpenAjax group and is working hard on being more compliant to others:
One thing that may make you feel a little better about this is that I'm personally involved in the IDE working group of OpenAjax, and metadata and IntelliSense interoperability are at the heart of the debate, so you can count on me to be a strong advocate inside of Microsoft to get those features in as soon as possible.