I frequently place an UpdatePanel around a paged GridView with inline editing.   During OnRowUpdating, the object is saved.   If the save was not successful, I like to show an alert box with the error message.  

I recently migrated from the Atlas May CTP to ASP.NET 2.0 AJAX.   The following code worked with the May CTP, but, from my research, has not worked since July CTP:

Page.ClientScript.RegisterStartupScript(typeof(string), "alertScript", "alert('Error Message');", true);

However, our pals at Microsoft have given us something just as good.   I found it by mistake while trying to put in a hack involving checking the value of ScriptManager.GetCurrent(Page).IsInAsyncPostBack.  Here is the AJAX-friendly equivalent:

ScriptManager.RegisterStartupScript(this.updatePanel, typeof(string), "alertScript", "alert('Error Message');", true);

Hope this helps someone.  There are many blog posts, with many more comments, with people compaining about this.