Actually, you really don't want to use Memcache with Liferay. I understand there's a lot of interest in memcache due to the hype surrounding it. However, memcache isn't suitable for all types of systems.
Unlike ehcache, memcache is not partitioned. So you don't have a separate cache per entity within Liferay. Liferay has over 300 entities that get cached. If you want to use memcache, you would need to create a dedicated memcache cluster for each of the 300 entities. That's a lot of overhead. You would be better off using the out of box ehcache + Terracotta as a distributed cache store.
Where memcache would be appropriate is to serve as a cache for specific objects. For instance, you want to use memcache as an EntityCache for Liferay User objects. The suggested way is to implement an adapter into Ehcache which would allow you to back the cache via memcache.
