Queries asked by almost every MOSS resource:

Integrating Ajax with MOSS:
http://sharepoint.microsoft.com/blogs/mike/Lists/Posts/Post.aspx?ID=3

Using AjaxControlToolkit in MOSS:
http://weblogs.asp.net/jan/archive/2007/02/26/using-the-ajax-control-toolkit-in-sharepoint.aspx

Smartpart for MOSS:
http://weblogs.asp.net/jan/archive/2007/02/22/smartpart-for-sharepoint-asp-net-ajax-support.aspx

Example: Ajax-enabled Grid in MOSS:
http://sharethispoint.com/archive/2007/02/28/Using-a-SPGridView-inside-an-ASP.net-Ajax-UpdatePanel.aspx