All ASP.NET developers should know by now that Microsoft is officiall supporting jQuery as part of Visual Studio 2008 (and beyond).
Well I've finally got to doing a project where I'm doing some very heavy AJAX implementations, and since it's a .NET build MS AJAX is already part of the loaded scripts, so I'm using jQuery in a supplimentary manner.
And loving it!
The jQuery vs-doc file makes life a so much easier, full intellisense support with code documentation.
Writing ASP.NET client controls that use both MS AJAX and jQuery is a snap too. When using Sys.UI.Control it's very simple to get a jQuery reference for the current control (psst - var j = $('#' + this.get_element().id)).
And the future is looking even more exciting - http://weblogs.asp.net/bleroy/archive/2008/12/09/microsoft-ajax-client-templates-and-declarative-jquery.aspx.
Client side templating, ASP.NET back-end power, the future of AJAX development with ASP.NET is looking very exciting!