I just tried and was able to install memcached-1.0.1 using both ree and ruby1.9.2dev (via rvm). Can you try with the latest rvm and see if this makes a difference? Also check your toolchain (I'm on g++-4.2.1 that was bundled with mac developer tools, I believe). I have a system libmemcached in both /usr/local/{include,lib} and /opt/local/{include,lib}, and these aren't causing issues (and from your errors, it doesn't look like you're trying to link against the wrong library either).
I just tried with ruby-head: ruby 1.9.3dev (2010-10-29 trunk 29623) [x86_64-darwin10.4.0] and it's working as well.
How are you installing the gem? just gem install memcached
?