Here's a court judgement you should be aware of. It has forced a former employee to give up details of his LinkedIn.com business contacts built up while employed by his former company. This decision highlights the growing tension between businesses encouraging staff to use social networking sites and then claiming the contacts are theirs when a worker leaves.
In a previous incarnation I agreed a person could take their contacts with them when they left but not copies of their emails. What any organisation needs is a clear policy. One that sets out the difference between contacts essential for professional development and those which would allow a leaver to set up a business at the employers ' competitive' expensive.