Does IE8 change the session id when switching to compatibility mode?  This would produce the behavior you are experiences.

> Does IE8 change the session id when switching to 
compatibility mode?

No. Having tested this, the VALUE for cookie named PHPSESSID 
was the same before and after toggling compatibility mode 
but the users session data was cleared (i.e. the user was 
logged out).

Is the PHP memcache client session mechanism tied to the 
browser's user agent string in any way?

Nothing is tied to the users browser agent.  Does the user get logged out when using the standard session save handler?

> Does the user get logged out when using the standard 
session save handler?

No, if I switch to using files for session storage, toggling 
compatibility mode doesn't cause a logout. However, I'm not 
exactly comparing like for like because I can only test this 
on dev which is a bit different from production.

It may be worth mentioning that we have redundancy and 
failover support switched on on prod:

memcache.hash_strategy = consistent
memcache.allow_failover = 1
memcache.session_redundancy=2
session.save_path="tcp://192.168.0.1:11211, 
tcp://192.168.0.2:11211"

For more details about the config see my blog post:
http://www.bitvolution.com/session-sharing-in-php-the-easy-
way

There have been reports that the session_redundancy setting has a problem.  Please try updating the following as a test to see if the resolves it:
memcache.session_redundancy=3

I opened /etc/php.d/memcache.ini and changed

memcache.session_redundancy=2
to 
memcache.session_redundancy=3

and restarted Apache. I Logged in to our website and toggled 
compatibility mode in IE8 - Unfortunately, no luck - the 
session was still lost and my user was logged out.

Thanks for the quick response.  Can we try one more setting change?  memcache.allow_failover = 0

I opened /etc/php.d/memcache.ini and changed

memcache.allow_failover=1
to 
memcache.allow_failover=0

and restarted Apache. I Logged in to our website and toggled 
compatibility mode in IE8 - Unfortunately, no luck - the 
session was still lost and my user was logged out. Note: I 
didn't restart memcached in between the config change 
(because then our website users would be logged out) - let 
me know if a memcached restart is needed as well as a Apache 
restart.

Please try to reduce the amount of differences between your testing and production environments. I.e. create a test env with multiple frontends and failover.

I'd guess the problem exists in the code/config of your production system, not memcache, so please try to reproduce it or dig deeper.

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