On Elijaa.org there's an interesting post (from back in May 2010) about using the "cas()" command in the memcached PECL extension to update the cached value of an object while preventing collisions with other scripts that might be updating the same value.

A series of commands is not atomic. If you issue a 'get' against an item, operate on the data, then wish to 'set' it back into memcached, you are not guaranteed to be the only process working on that value. In parallel, you could end up overwriting a value set by something else. [...] But PECL Memcached come with a useful function : cas().

The "cas()" function uses a checksum pulled out when the data is uplled using "get()". This token along with the updated data is passed into the "cas()" method that does the additional checking for you. A code snippet is included to show you its use.