If I quey the cache on the client to get a list of persons, insert a person and after that query the cache again does the cache contain my own inserted person? 

I would also be interested if the same is true for the server version. So if it is true it would mean that on the server I could always query the cache as long as there is no other application directly inserting into the database.

There are 2 types of caching in DevForce,

1. Query caching -  http://drc.ideablade.com/xwiki/bin/view/Documentation/query-cache - http://drc.ideablade.com/xwiki/bin/view/Documentation/query-cache

and

2. Entity caching (EntityManager's cache) -  http://drc.ideablade.com/xwiki/bin/view/Documentation/start-entity-cache - http://drc.ideablade.com/xwiki/bin/view/Documentation/start-entity-cache

They both determine DevForce caching behavior.

The answer to your first question, is yes. When you add that person to the EntityManager, it will be stored in the EntityManager's cache.

There's no difference in caching behavior for client or server's version of EntityManager. So, yes, if you have queried all data into your server version of EntityManager, then you can always work with its cache.