Решение 1

<%@Page%>
<script language=javascript><!--
function getOb(name) {
    return document.all ? document.all.item(name) : document.getElementById(name);
}

function setStyleVis(name,s) 
{
    var ob = getOb(name);
    ob.style.display = (s==0)?'none':'block';
}

function disable(btn,txt) {
    setStyleVis(btn,0);
    setStyleVis(txt,1);
}
//--></script>

<script language="C#" runat=server>
    void Page_Load( object sender, EventArgs e ) {
        btnSubmit.Attributes.Add( "onclick", "disable('" + btnSubmit.ClientID + "','submit_text');");
    }
<script>

<form runat=server>
    <asp:Button runat=server ID="btnSubmit" Text="Submit"/>
        <span style="font:10pt verdana;display:none;" id="submit_text">Processing,please wait...</span>
</form>

Решение 2

<%@Page%>
<script language="C#" runat=server>
    void Page_Load( object sender, EventArgs e ) {
        btnSubmit.Attributes.Add( "onclick", @"if (this.getAttribute('ac')!= 'true')
{this.setAttribute(ac,'true');} else {return false;}");
    }
<script>

<form runat=server>
    <asp:Button runat=server ID="btnSubmit" Text="Submit"/>
</form>