Would it not be great if you could write something like:
And be assured that EntityManager.merge() would fail even if somebody manually replaced the entity id somewhere along the way? Wouldn't it be equally cool if you could just do EntityManager.find(Account.class, null) to fetch the right Account for the currently logged-in user? If securing data instances have been causing gray hair for you before and you happen to be using JPA, you should definitely checkout Tynamo's latest module, tapestry-security-jpa.
On a related note, if you happen to live in SF Bay Area, I'll be talking about ERBAC, federated accounts, tapestry-security and using Shiro in modern Java web applications in an upcoming Shiro JUG meet-up this Wednesday, graciously sponsored by Stormpath, Inc.!