Well here is some smarter tal.

    <tal:if tal:condition="exists:request/env/QUERY_STRING">
       <input
tal:condition="python:request.env['QUERY_STRING'].find('@action=logout')
== -1"
	      type="hidden" name="__came_from"
	      tal:attributes="value
	     
string:${request/base}${request/env/PATH_INFO}?${request/env/QUERY_STRING}">
       <input
tal:condition="python:request.env['QUERY_STRING'].find('@action=logout')
!= -1"
	      type="hidden" name="__came_from"
	      tal:attributes="value
	      string:${request/base}${request/env/PATH_INFO}">
    </tal:if>
    <input type="hidden" name="__came_from"
	   tal:condition="not:exists:request/env/QUERY_STRING"
	   tal:attributes="value string:${request/base}${request/env/PATH_INFO}">


it looses the page you are on when you login after logging out,
but I figure it serves you right for logging out of roundup 8-).

Anybody want to try this code?