The "Remember Me" can be changed from Goldfish to Elephant mode by setting the timeout to a longer value in the forms authentication section of the web.config.  It is only 60 minutes by default.

This seems to get reset every time DotNetNuke.com does an upgrade.

Back to the original thread. I have serious doubts that someone actually came to DotNetNuke.com and was automatically logged in as some other user, then decided to post the very first message on that account, and then to have the supposed original account holder notice it right away and say "hey someone else posted under my account".

It's a serious accusation though, so it would be nice if someone with the power to look into it would do so.

Ok, that sounds like a legit reason for the immediate response. 

You have probably already done this, but if not please change your password.

My curiosity is raised.  How long has it been since the last time you were logged in, before getting this message today?