You could create a usercontrol with a button that fires FormsAuthentication.Signout();

Or if it fits your demands you can also use the asp:LoginView, wich contain logout.

asp:LoginView -> asp:LoginStatus that is.

Thanks for the replies.  I've tried this, (adding the ascx and dll files to Umbraco and creating a macro) but nothing happens:

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="memberLogOut.ascx.cs" Inherits="memberLogOut.memberLogOut" %>

<script language="javascript" type="text/javascript">
// <!CDATA[

    function Button1_onclick() {
      FormsAuthentication.Signout();
    }

// ]]>
</script>
<p><input id="Button1" type="button" value="Log out" onclick="return Button1_onclick()" /></p>

Can anyone point me in the right direction?

I should add... ideally I'd like to check the log-in status to make sure the user is logged in before giving them the log-out option.

Doh!  Didn't spot that the member package actually has a log-out functionality able to be built in via the macro parameters.  Sorted.

is working on a reply...