Sweetwater, an online retailer of music equipment, just recently started this blog about all things guitar. This morning, I opened my inbox and alas! what did I find? It was their latest newsletter, with a pretty cool exercise for increasing your picking speed. They call it "Speed Bursts", and you can check out the whole article on their site:
http://guitarsandgear.sweetwater.com/2010/02/technique-speedbursts/