The PHP::Impact blog has a recommendation for those using the memcache functions in their PHP applications - be sure your hashing strategy matches what your script does.

If you are using the Memcache functions through a PECL extension, you can set global runtime configuration options by specifying the values within your php.ini file. One of them is memcache.hash_strategy. This option sets the hashing mechanism used to select and specifies which hash strategy to use: Standard (default) or Consistent.

The recommendation is to set it to consistent to allow for the most flexibility on adding and removing servers from the caching server pool without the need for outside intervention.