The new MemCache stuff needs a perl module "MemCachedClient.pm", located in $LJHOME/src/memcached/api/perl. Since this path isn't mentioned anywhere, obviously the thing couldn't find it.
I solved the problem by adding the line
use lib "$ENV{'LJHOME'}/src/memcached/api/perl";
just above
use MemCachedClient;
near the top of $LJHOME/cgi-bin/LJ/MemCache.pm.
Brad: Might it be worth actually adding that in CVS? Or how else is it supposed to work?