As found out through PMs, it was a joomla 1.5 install:

- Joomla 1.5 beta 2 is not compatible with CB 1.0.2, as we stated after tests on first page of discussion thread of joomla 1.5 announcement.

- Joomla 1.5 SVN corrects the biggest joomla bugs preventing compatibility.

- there might still be a few bugs around login management in joomla 1.5 svn, that will be addressed in the following weeks.

At Joomla! 1.5 beta 1 release, there was time to make sure Joomla stayed compatible with CB. At Joomla 1.5 beta 2 release, there was unfortunately not enough time for making that sure.

We are working with Joomla core team to make sure that Joomla 1.5 RC will be compatible again.

You might want to checkout from svn or download a nightly build of Joomla 1.5, and redo the tests.

Your report with that here as well as in Joomla 1.5 Q&T forum on forum.joomla.org would be very useful.

haltonsc wrote:

This happens in firefox as well, if you don't logout and just close the browser

Joomla 1.00.12, siteground, haltonsc.com
Latest download.

Registered on your site to try to see what's happening with cookies and so on. Waiting for moderator approval

Once problem solved you can delete my account on your system

Two things to check on your side:
1) is server clock correctly set ?
2) is server time-offset correctly set in global site configuration ?

haltonsc wrote:

This happens in firefox as well, if you don't logout and just close the browser

Joomla 1.00.12, siteground, haltonsc.com
Latest download.

Just checked with safari 2, and it auto-logs out ok. Will check with safari and ie next.