As you can read on the Microsoft Product Feedback Center the idea of adding AJAX support to Whidbey is to late. But Microsoft will think about it:
*Thank you for your suggestion. AJAX is defininitely gaining attention and is something that the team has given thought to. Considering where we are in the product cycle, I hope you'll understand that it wouldn't be possible to do this in Whidbey but we'll evaluate the suggestion for a future release. We'll update the status of this bug further after we've finished triaging it.
Rich Ersek The Web Platform & Tools Team*
Please vote at the web site for AJAX support in Whidbey ! You can download the free Ajax.NET Library at: http://ajax.schwarz-interactive.de/ .