As you suggest, either openId or SSO would be suitable.
See my post on 29 July 2008, 12:50 in http://groups.google.co.uk/group/comp.lang.php/browse_thread/thread/b682ed2a8b7fbb3f/f5f27f120e264fa0?hl=en&lnk=gst&q=single+sign+on+symcbean#f5f27f120e264fa0