When I set up my first MediaWiki (nice work, btw!), I got "Turck MMCache not installed", even though I had just installed it. (I assume some app or daemon needed to be reloaded to detect it -- which one?)
Later, when I installed a second wiki, Turck MMCache was detected, and I activated it. Seeing the result of the activation in LocalSettings.php, I uncommented the same line for the other wiki --
$wgUseTurckShm = function_exists( 'mmcache_get' ) && php_sapi_name() == 'apache';
The effect was a messed up cache function -- the second wiki's Main_Page would consistently display in the first wiki. If I selected [Edit] and Preview, I would see the first wiki's real Main_Page intact, but saving it would revert the display to the second wiki's front page.
Nikerabbit on #mediawiki irc diagnosed it and I fixed the problem easily -- whew. But how do I properly activate Turck MMCache for a wiki that is already installed?
Dave