Apparently there is a bug in libmemcached where connect timeout does not work when OPT_NO_BLOCK is set as well.
The bug and related blocking bugs are discussed here:
https://bugs.launchpad.net/libmemcached/+bug/583031
We confirmed this in libmemcached version 0.43
Be sure to test some memcache servers in your pool being unavailable (no route to host and memcache service down) to ensure that timeout settings are working correctly.