I don't know what happened. When I moved my scripts to a class library project, I found that Telerik intellisense stopped working.
I registered the Telerik.Web.UI assembly in the GAC but this didn't solve the problem.
I had to extract the script files from the assembly and store them on my disk for intellisense to work. Here's the contents of my _refernces.js file now:
I hope this doesn't violate the terms of use.