For anyone who hasn't seen this yet, Petzl has posted a new-and-improved technique of belaying a lead climber with a Grigri, explained in a little movie posted on their Grigri product page http://en.petzl.com/petzl/SportProduits?Produit=203#. In a nutshell, the new technique encourages the brake hand to cover the brake strand continuously, lessening the possibility that the belayer will lose control of it during a fall. For me this new technique feels not only more simple and secure but allows a faster feed during quick clipping than the "old" technique, which is shown in this diagram on Climbing.com http://www.climbing.com/print/techtips/ttsport243/.
Also, this is a very good article on the challenges of using a Grigri safely, http://www.thebmc.co.uk/Feature.aspx?id=1547.