From Jan Tielens:

 When started talking about ASP.NET AJAX the audience (+300 people!) started asking some questions, in my turn I asked Scott about using ASP.NET AJAX in SharePoint 2007. Scott confirmed Daniel’s statements (not supported, only use the the client side scripts, server-side controls don’t work, etc), but more interesting; he told the audience that ASP.NET AJAX will be supported in SharePoint 2007. How is Microsoft going to pull off this trick? ASP.NET Ajax will be supported in SharePoint when we release a service pack for SharePoint, we will ship it later this year.Great!!

See also Daniel Larssons blog on how to use Ajax with SharePoint (the unsupported way)