The main obstacles we experienced is in projects where you have moving targets - requirement changes every day. It requires a very good shape of discipline of all stakeholders in the project. See how we started...
In my previous blog post, I was asked how to implement Scrum in the first steps. My personal experience is that it's better not to directly begin with all the methods Scrum provides. Scrum requires changes in your internal business processes, so it's better to move ahead step by step instead of trying to get everything done until tomorrow (you see, incremental steps.
He describes each step of the way, beginning with a meeting for the first sprint followed by a short sprint time - 5 days. They even mention a PHP app they've worked up to help track and manage this kind of development - a module for their PHProjekt software, the Scrum Addon.
