I have a Liferay portal 5.2.3 version configured for NTLM. So clicking on the dock Welcome -> Sign-in signs me in without prompting for a password. I have copied over the same settings into SO 1.5b1 using the Control Panel. However, clicking on the Sign-in link a the top right prompts me for login.
Looks like a skin issue? In liferay, the Sign-In links to http://xxxxxx/c/portal/login?p_l_id=10139 while in SO it is http://xxxxxx/web/guest/login