It's Feb 2012, and I just spent half a day tracing a weird and sporadic problem back to Memcached extension (vers 1.0.2), which would randomly return SYSTEM ERROR and fail to either get or put data into unix memcached. Made one single change from Memcached extension back to Memcache extension and problem instantly vanished.
A year or three ago, I had changed from Memcache to Memcached for some other reason.
Now I'm on a mission to transcend the Memcache/Memcached fiasco. I'm going to try APC's variable cache, and if that doesn't work better than just revert to using a key:value stash collection in Mongo.
Basically, the Memcache / Memcache driver fiasco, which has been going on apparently for years now, has ruined serious use of unix memcached on PHP...unless you're huge and can afford to create your own MemcacheX driver...