My understanding is that once you enable the persistent object cache, your transients are no longer stored in
wp_options but in the cache.
Once I installed Mark Jaquith's APC Object Cache Backend, calls to
get_transient( $key ) no longer return the transient in question.
Where should I look next?
EDIT: One very important piece of evidence is that AS SOON as I renamed the
object-cache.php file, everything started working again.