By the way, I did this in vb:

Dim sb As New StringBuilder
sb.Append("if (typeof(Page_ClientValidate) == 'function') { ")
sb.Append("if (Page_ClientValidate() == false) { return false; }} ")
sb.Append("this.value = 'One moment...';")
sb.Append("this.disabled = true;")
sb.Append(Me.Page.GetPostBackEventReference(Me.btnSubmit))
sb.Append(";")
btnSubmit.Attributes.Add("onclick", sb.ToString())

And it said GetPostBackEventReference is obsolete, and I should use ClientScript.GetPostBackEventReference.

I've done this before and it is fine as far as it goes.   But now say that I have 2 buttons on the page and I need to disable both on the click of one or the other.  "This" works well for the one pressed, but obviously won't work for the other and using the find by element id has issues because of the way the page is rendered.  Any suggestions?