I'm going to piggy-back on Feras' discussion about SFDC moving from Leads & Contacts to People a bit (I wholeheartedly agree that SFDC needs to make that change BTW).
I'm curious how you handle existing Contact records when a person moves to another company. More specifically, when an existing Contact moves to a company that you want to sell into, but have not qualified (i.e. all you have is Leads for that company). Do you deactivate the existing Contact and create a new Lead? Do you create an Account for a prospective customer company that you've not qualified and then move the existing Contact record? Or is there another approach that I'm missing?