I just went through the following sequence:
Go to superuser.com.
Compose a post with text and tags.
Scroll down to post, and realize I'm not logged in.
Open a fresh tab on superuser.com.
Click on the login tab, and maybe select OpenID, but before I can start entering data (my OpenID is http://JonathansCorner.com handled by http://JonathanHayward.MyOpenID.com, and I believe I clicked "Remember me for three weeks" on the MyOpenID login page), the system logs me in and presents a welcome back message at the top.
Go to the tab that I had composed my question in.
Hit "Reload" and be presented with a dire warning about leaving a page that has state.
Click the "Yes, I really want to reload after your warning about losing data" button.
Page is reloaded perfectly, with all of the state in question text and tags preserved.
Hit "post" button, and flow from here on is as if I had logged in first and then posted my question second.