wpmu – enabling wp-super-cache doesn’t “stick”

I’m trying to get wp-super-cache running on a WPMU installation, and have installed the plugin into mu-plugins. I’ve added the line define(‘WP_CACHE’, true); to the top of wp-config, and have verified that all cache directories and php files exist as expected in wp-content.

But… When I go to the Site Admin section and hit the WP Super Cache tab, then check the “On” setting and Update Status, it doesn’t seem to stick. Whenever I come back to the page, it’s been set to Off again.

I don’t see anything in the Apache error log.

Specs:

WPMU 2.6.1
wp-super-cache 0.71
Server running RHEL5 (PHP 5.1.6, Apache 2.2.3)

Any ideas?