With plans for APC to be included by default with PHP6, I would anticipate that more folks may start using it. Currently, under SERVER->PERFORMANCE we have plugin options to take utilize internal, eaccelerator, and memcached. It would be good to add APC to that list.
Martin Langhoff mentioned this as a possibility in: http://moodle.org/mod/forum/discuss.php?d=75689#p357136 .
APC uses shmem by default. I'm attaching my production servers apc.php page should it be helpful.
I'd be happy to help with testing. Peace - Anthony