See forum http://www.avaje.org/topic-128.html
An example:
Query<Product> subQuery = Ebean.createQuery(Product.class) .select("sku") .where().idEq(4).query(); List<MinCustomer> list = Ebean.find(MinCustomer.class) .where().in("name", subQuery) .findList();
select c.id, c.name, c.notes from o_customer c where (c.name) in (select p.sku from o_product p where p.id = ? )
Credit goes to Mario for this enhancement and the code.
Code to support this is now in HEAD.