ok.. I think I'm the only one having this weird problem.
I installed brand new WP 3.4.1 with ALL privileges to the DB
installed brand new xampp-win32-1.8.0-VC9 on my local host setup a fake domain "some.dev" on windows' host file.
did a svn of your plugin 2.0beta4 [svn revision: 551921] to the plugin directory. there is no other plugin other than the default plugins which comes from WP
activated and tested the login, NOPE, ZIP, no warning or error message and no logout.
I did a save of options on the settings page for this plugin, in the hope that some settings may not have been saved.
NOPE that didn't make the logout work too.
I have a test site, http://anjanasglitz.com/
which runs on WP 3.3.2
your plugin 2.0beta4 [svn revision: 551921]
There is absolutely nothing about logout as well.