This fix has measurably helped our db performance:
Expunging the cache of dropped table pages used to be one of the largest causes of blocking in our db's. It used to be right up there with the blocking caused by ROLLBACKs (see my earlier post). We've now been running the Percona build of 5.1.47 for a while, and things are way better. In fact, purging the page hash (the data structure that lets the db look up where a given page resides in the cache) of deleted table page entries now causes more blocking than cleaning up the cache itself.