New in version 0.53 (which yes, I really should renumber into 1.X at some point in the near future) is memcached_exist().
Ever wanted to find out if a key existed but didn’t want to have to fetch the object?
Well now you can do this. It works by seeing if an add can be done on the key (the add though is dated in the past, so any write afterward will expire it).
You can currently grab the code via bar on Launchpad.