(Edit - 2012: in the meantime I use a 2 mm Noene insole. With them you don't notice the sudden change mid foot. I do this with all my shoes that have a heel drop: Brooks Pure Flow, Pure Grit, Inov-8s)
Recently, I had to do more of my runs on the road. Since I suffer from a slight chronic sprained ankle I needed a some more cushion beneath my feet. Getting a regular cushioned monster shoe was not an option, I simply can't run with a significant heel drop anymore. If Inov-8 just made a zero drop shoe with 2 or 3 arrow cushioning. Not everyone seeking cushion is a heel striker. Altra's Instinct looks really promising, however, it's not available in Europe.
Therefore, I've cut a cushioned insole (Sof sole Thin Fit) and put the front part below the original insoles in my shoes. The sudden change of height below the mid of my foot is quite noticeable when standing or walking. However, since I'm a forefoot striker it does not bother me.