memcachedb is not distributed, meaning that you can only scale vertically (i.e. by buying bigger machines). With LightCloud you can scale by adding extra servers to the system (i.e. horizontal scaling).
This said, Tokyo Tyrant performs better than memcachedb and offers more features (such as master-master replication and Lua scripting).
I posted a comment a while back on Tokyo Tyrant vs. memcachedb: http://news.ycombinator.com/item?id=480055