Recently installed aMember and integrated it with the latest version of WordPress (2.7.1). All day I was having issues with single signon and finally fixed it this evening. Thought I would share for others having similar issues. I ended up changing the values in AUTH_KEY, SECURE_AUTH_KEY, and LOGGED_IN_KEY to a combination of letters, numbers, and limited special characters. Using the WordPress generator it had included characters such as \ " % ^ &, etc. Not sure exactly which of these were causing issues, but I would assume the backslashes and double quotes are prime suspects. Anyway, hope that this helps others having similar issues.