Hi Paul & everyone,
I'm having very similar problems on certain machines.
1) I can't logout once I'm logged even (even with full page refreshes)
2) I can't view the admin/configure.php file without being asked to login again, and that then just loops to the login page again.
I've tried editing the htaccess file as you describe, setting my local value of php_value session_cache_limiter to nocache. I can see the change made in phpinfo, but it's not made any difference on that machine.
I'm beginning to think it's an issue with my work network's proxy server (although it happens with a colleague as well who's accessing via NTL's cable network). Or it might be a firewall issue (my work laptop - with the problems - uses Macafee desktop firewall (which I cannot switch off or reconfigure), my colleague uses Norton & my home PC (which is working fine) uses ZoneAlarm.
PS. I've already had some correspondence in http://moodle.org/mod/forum/discuss.php?d=6783 but I thought I might try to widen it a bit.