You want to put the LogIn button in header section then why dont you keep the actionlink tag in header tag of div-role ?

<div data-role="header" class="ui-bar">
        <%: Html.ActionLink("LogIn", "LogIn", "LogIn")  %>
    </div>
Check out some examples:

http://www.asp.net/mvc/tutorials/aspnet-mvc-4-mobile-features

http://weblogs.asp.net/gunnarpeipman/archive/2010/10/27/building-mobile-web-site-with-asp-net-mvc-and-jquery-mobile.aspx 

http://weblogs.asp.net/jalpeshpvadgama/archive/2011/10/09/asp-net-mvc-4-0-mobile-template.aspx

Thanks,

Anindita

http://anindita9.wordpress.com