If the source of the submit is disabled, i.e. your button, the page won't submit. Simply delaying the disable for a tad may work:

Code:
function showSending(button) {
	window.setTimeout("button.disabled = true", 50);
	
	var linkID = document.getElementById("sending");
	linkID.style.visibility = "visible";
	return true;
}
</script>

<asp:Button ID="btnSending" runat="server" Text="Sending" OnClientClick="return showSending(this)" OnClick="btnSending_Click" />
<div id="sending" style="border: solid 1px green; padding: 5px; width:55px; font-family:Verdana; font-size:12px; visibility:hidden;">Sending..</div>