Are airline offsetting schemes all they're cracked up to be?

Most airlines now offer passengers the option to offset their share of the flight's carbon emissions with carbon credits. Air New Zealand's scheme is called "Fly Neutral". A one way trip between Auckland and Wellington creates 62 kilograms of CO2 for one passenger, and the airline says for $1.50, that can be offset by buying carbon credits from "global sustainable energy projects" and contributing to restoration of biodiversity here.