the util_memoize cache is a size limited cache so will not get too large and does LRU flushing. It might make sense to make this a seperate cache from the util_memoize cache and make it a little smarter (and possibly think about just bulk flushing the entire cache on "dangerous" ops like revoking admin).

Right now the permissions cache is really not ready for prime time and I would discourage people from using it unless they either a) don't care about security, b) don't really ever change permissions or c) are extremely careful to look at how their particular site works and think through the consequences of a permissions cache that does not really flush properly.