I seem to remember someone (Monte?) saying that the use of freelancers goes in cycles - you have times when everyone is on staff, and then times when lots more freelancers get used.
Currently, Wizards has a system where they get a lead designer of a book on staff, and then might contract out work for that book.
If I have my facts straight, Stephen Schubert is one of the new Wizards employees (he worked on Heroes of Battle and Magic of Incarnum - he comments on the latter, "it's an exciting book, and something entirely different than anything you might expect. It was definitely fun to write. We can elaborate more once the catalog is released."
Cheers!