ScottGu broke the news last night that Microsoft is making a shared CDN available for the purposes of hosting the AJAX scripts. The full details of the scripts that are supported right now are available here, but basically it’s ASP.NET AJAX 4.0 Preview 5 (which just came out) and jQuery 1.3.2.
If you’re using ASP.NET AJAX on the server side, you can just tell the ScriptManager to use the CDN by setting the EnableCdn property to true. I find this implementation a little misleading because it only applies to the scripts within System.Web and System.Web.Extensions. If you want support for other scripts you would need to put them on your own CDN and still hook the ScriptManager::ResolveScriptReference event and set the ScriptRefrenceEventArgs::Script property accordingly.