Just migrated to pressflow and now users can't log in.. any ideas?
Not sure if that's going to be of any help for you, but I had kind of similar problem - on staging site, after upgrade from Pressflow 6.16 to 6.26, users were not able to log in anymore - or actually, according to Watchdog, they were logged in, but still being redirected to log in form.

No warning like yours though, although I assume that in your case they we caused by your admin account already logged in.

At this time prod site was still running Pressflow 6.16.

What helped in my case was changing $cookie_domain value in settings.php file on the staging server, which "unlinked" staging and prod cookies. 

Essentially, before both staging and prod sites were using the same $cookie_domain (let's say it was mydomain.org), and everything started to work fine after changing it to stage.mydomain.org on the staging server.

Perhaps the problem is with the different way of handling cookies, and once our Pressflow is updated on prod server too it will again work fine with the same $cookie_domain, but this is still to be tested and confirmed.